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On the Action of Nitric Acid on Picramic Acid.

fact to which I am about to draw attention is, I am inclined to think, quite original, and will go far to explain away the mystery.

If we double the true formula of the oxalate, thereby making its equivalent 180 instead of 90, we see at once that a very curious numerical relation exists between the equivalent numbers of the three bodies engaged. The following Table will render intelligible what I

mean to convey :

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Thus, if one equivalent of protoxide of iron is wanting its place is exactly filled up by an additional equivalent of oxalic acid; so that, supposing this alteration to have been made, we arrive at the formula FeO, 3C2O3+4HO, which is identical with that last proposed by Dr. Phipson. Again, let us take another view of the matter. Dr. Phipson, all through, owing to the miscalculation of his results, has found only half the real quantity of protoxide of iron present. Now, if the salt be analysed by simply igniting it and calculating all the loss as oxalic acid, it is obvious that we must arrive at the formula FeO,4C2O3, which is that first proposed by Dr. Phipson.

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mention this to show my reason for using one salt in preference to another.

The equation which Döbereiner has given for expressing the decomposition under the influence of the solar rays is, doubtless, quite correct when the per-salt is in solution; but when the per-salt is in the solid state and exposed to the rays of the sun, it blackens. If the blackened powder be now immersed in water, the yellow proto-salt is immediately produced. This fact was first noticed by Dr. Phipson, and I have frequently had occasion to corroborate the observation. It appears to me to be very probable that this black substance is the protoxalate in the anhydrous condition, which, on coming in contact with water, assimilates two equivalents, thus becoming the ordinary hydrated oxalate. I have not yet had time to verify this supposition.

With regard to the reaction between ammonio nitrate of silver and protoxalate of iron, I think the following nature of the decomposition which occurs. equation will show more clearly than words can the The experimental grounds on which it is based have been already given in the first part of this papers :—

2(FeO,C2O3)+3(AgO,NO ̧ † 3NH10)

T

FeO3+ Ag+2(AgO,C2O3) + 6NH,O+ 3(NH,O,NO,.) I need only remark that the excess of ammonia holds the oxalate of silver in solution, from which it is precipitated on neutralising the free alkali.

On the Action of Nitric Acid on Picramic Acid, by M. C. LEA.

Curious to say, the extraordinary coincidence above pointed out has never yet been mentioned by any of those who have from time to time given their attention ON this point very conflicting statements have been to the study of the chemistry of these salts. From what made. Girard and Pugh respectively state that picric I have said, it will be obvious that, although it is sug- acid is reproduced by the oxydation of picramic acid by gested how the errors may have occurred, we are still at a loss to know by what means the gap of 36 parts in nitric acid. A similar statement is made by Kolbe in the whole equivalent of the salt has been filled up; for his " Lehrb. d. Org. Chemie" (authority not given). In we are distinctly told by Dr. Phipson that the oxalic acid a paper published several years since, on picric acid, I was calculated from the amount of carbonic acid produced expressed a similar opinion. On the other hand, Wöhler on the combustion of the salt with oxide of copper. There-identical with picramic) was not reconverted to picric stated that his nitrohæmatic acid (now known to be fore, an equivalent quantity of carbonic acid ought to have been found corresponding to the 36 parts of oxalic acid. Whether this was the case or not, we have not sufficient evidence to show, beyond the mere statement, indirectly made, in the calculation of the results. I am inclined to think that if Dr. Phipson will re-consider his former calculations, paying due attention to the sources of error which I have mentioned above, he will find that the true formula for the oxalate of iron is, FeO,C2O3 + 2HO. 2. Certain Reactions of the Oxalates of Iron. -Under this head, I merely wish to draw attention to the theoretical explanation of the change which occurs on exposing oxalate of peroxide of iron to ordinary light; and likewise to demonstrate the nature of the reaction which takes place on treating protoxalate of iron with ammonio-nitrate of silver.

According to Döbereiner, the following equation represents the change which takes place on exposing solution of peroxalate of iron to the light :

C12Fe,O242(C,Fe2O)+4CO2.

=

The sensitive surface which I use in printing by light is the ammonio-ferric oxalate, which has no solvent action on the deposit of proto-salt formed under the influence of light; whereas, if the plain peroxalate were used, the moment water was applied to the paper it would dissolve and attack the oxalate of the protoxide, over which it exerts considerable solvent power. I only

acid by the agency of nitric acid. Gerhardt, too, in
quoting the first opinion, puts a note of interrogation
differences of opinion have induced me recently to re-
after it, as if to express a contrary conviction. These
examine the subject, and have led to the conclusion that
the substance formed is not identical with picric acid.
The following were the reactions observed :-
to a dark brown solution. By fifteen minutes boiling
Picramic acid readily dissolves in strong nitric acid
this becomes clear bright red. If then saturated with
potash, quantities of nitrate of potash crystallise out,
with much brown varnish, but no trace of picrate. After
one hour's boiling the colour of the solution is consider-
ably lighter, the results much the same.

After four hours' boiling the colour of the liquid was bright yellow. It was evaporated in the water bath and gave a crystalline substance mixed with much resinous matter. To remove this it was dissolved in as small a quantity of cold water as possible, filtered and mixed with half its bulk of strong sulphuric acid. On cooling might easily be taken for picric acid mixed with resinous a crystalline reddish yellow substance separated, which impurity. But neutralised by ammonia and heated with sulphydrate of ammonia it gave no indications of the presence of pieric acid. Tested with cyanide of pe sium the results were the same, By spontaneous

5 This is the formula assigned to the compourd 1

ration of the solution of the substance in ammonia, fan-centrated sulphuric acid began to react on this substance." shaped groups of hair brown needles were obtained. Binitronaphthaline resists the action of sulphuric acid Analysis of these showed conclusively that they consisted at a very high temperature, however, at about 300°; the of oxalate of ammonia disguised by organic matter. colour of the solution, at first slightly yellow, deepens After eight hours' boiling the liquid was pale straw more and more, becomes cherry-red, and finally brownishyellow, and by evaporation on the water bath yielded red, beginning at the same time to disengage a small a substance dissimilar from the former, bright yellow, quantity of sulphurous acid. The progress of the and coloured intensely deep red by cyanide of potassium operation is easily followed by taking up occasionally a after previous supersaturation with ammonia. But drop of the liquid with a rod, and dropping it into a treated with sulphydrate of ammonia, it gave no indica- glass of water. Thus at first a white milky precipitate tions of the production of blood red picramate, but is obtained, then a light violet; and finally, when the became greenish brown with production of a greenish colour is completely developed, a dark violet. precipitate. The presence of oxalic acid could not be detected.

These experiments appear to me to leave no doubt that picric acid is not formed by the action, either brief or prolonged, of nitric acid on pieramic acid, but that resinous substances are produced, accompanied after a time by oxalic acid, which at a later stage suffers decomposition itself. All these substances are, however, produced in very small amount, the greater part of the constituents of the picric acid passing off in volatile decomposition products.-American Journal of Science, No. 95.

TECHNICAL CHEMISTRY.

Contributions to the History of Naphthaline,
by M. J. PERsoz.

AFTER reading M. Roussin's interesting communication, on an artificial coloured product said to be identical with alizarine, I think I ought to make known to the Academy the results I obtained two years ago, while studying, with M. Martel, the derivatives of naphthaline.

Starting from the fact, established by us, that a mixture of commercial nitre and sulphuric acids, even in very variable proportions, will, when heated with naphthaline, readily yield coloured products, we have naturally been led to examine the action of concentrated sulphuric acid on the various nitrogenized compounds of naphthaline.

This is a very difficult study, however simple it may at first sight appear, because the least changes of the condition under which the experiment is performed, exercise a sensible influence on the results. The dye principle formed, possesses the property of madder in dyeing mordants; its colour varies from red to blue, and passes through all the shades of violet.

The blue was only obtained accidentally; and we are unable to state the precise conditions of its formation, though it appears to be due to a molecular change in the nitrogenized naphthaline compound, under the influence of a physical agent.

As the violet-blue tints are the most beautiful, we have devoted most of our attention to them, and have endeavoured to produce them; thus working in an opposite direction to that of M. Roussin, who is chiefly occupied with the reds. We soon found that binitronaphthaline, heated with sulphuric acid only, was best suited to our purpose. In his last communication to the Institute (Comptes Rendus, Vol. lii., p. 1033), M. Roussin says:"By making concentrated sulphuric acid react on binitronaphthaline, no reaction takes place. The binitronaphthaline is completely dissolved, when the mixture is heated to 250°, and the liquid takes hardly an amber colour. After boiling for a long time, the con

The substance is then taken from the fire, and left to cool, when it is poured into a proper quantity of water and boiled. The liquid, filtered whilst hot, is of a deep red colour, and deposits part of the colouring matter in a flaky state. Alkalies change it to violet red; and even when cold, silk was easily dyed violet by it. After being properly saturated with alkalies, and finally with a little chalk, it dyed mordanted cotton tissues with different shades, varying from lilac to black. The lakes with alum, tin, and lead for a base, are violet; those with iron for a base, were olive, and sometimes reached to black.

In fact, this solution does not seem to alter even during any length of time, in presence of sulphuric acid; though when in contact with air and excess of ammonia, it changes to brown in a few hours, depositing a black powder, which becomes blue when dissolved in alcohol, and red in acids.

The black mass proceeding from the precipitation of the sulphuric solution by water, contains a large quantity of colouring matter, which we were able to separate by means of M. Payen's digesting apparatus. This colouring matter has a beautiful gold reflection, is very soluble in alcohol and pyroligneous acid; but very little soluble in water, ether, benzol, and bisulphide of carbon. As we have before said, it has many chemical analogies with alizarine. In fact, according to whether a bath is slightly acid or alkaline, we can dye the iron mordants to the exclusion of alum mordants, and reciprocally. Moreover, the dyed tissues bear brightening with soap, carefully done, that is to say, progressively. Finally, the colouring matter readily sublimates under the influence of a high temperature.

It is evident, then, that with binitronaphthaline and concentrated sulphuric acid only, without making use of a reducing agent, as M. Roussin has done, a colouring matter may be obtained with marked analogies to alizarine in its chemical properties; however, the observations I have made during my operations, have led me to doubt whether it is possible, even while obtaining perfect red tints, to prepare in this way a colouring matter identical with that of the madder.Comptes Rendus.

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Royal Institution of Great Britain.

The

ducing new substances in that branch of science. success, however, which has lately attended its application, will tend greatly to increase its importance as a therapeutic agent. It has been used with marked advantage in the Manchester Royal Infirmary by several of its distinguished physicians and surgeons. Thus, Dr. Henry Browne has given it in solution in water in cases of chronic diarrhoea, with very satisfactory results. Dr. Roberts has applied it with very great success in the dose of one drop, in cases of vomiting, even after creosote had failed; he has also found it beneficial in cases of vomiting from dyspepsia, which disease is especially marked by pain after food. Mr. J. A. Ransome has used it for ulcers and other offensive discharges. Mr. Thomas Turner, in a note which he has communicated to me, speaks of carbolic acid in the following

terms:

"It may be advantageously used as a solution of one part of acid in seven parts of water, in fœtid ill-conditioned ulcers. It alters the action of the blood-vessels, causing a purulent instead of a sanious discharge, and destroys almost immediately the offensive smell of the secretion. The ulcers having a communication with carious bone, or even necrosis (where the bone is dead), it has in its diluted state a good effect when injected into the sinuses leading to the diseased bones. When there is mere caries or ulceration of the bone, it effects the healing process; and in necrosis it promotes the exfoliation of the dead portion. In gangrenous and all offensive sores it removes all disagreeable smell and putrescency, and may render the discharge innocuous to the contiguous living and unaffected tissues. In its diluted state, therefore, it is a great boon to patients labouring under that class of disease."

Mr. Heath, house-surgeon of the Infirmary, has used it with two parts of water as a lotion in sloughing wounds, and has found that in a short time after its application, it entirely arrests the sloughing process, and produces a healthy appearance.

Dr. Whitehead has used with advantage Dr. Robert Angus Smith's solution of sulphites and carbonates of lime and magnesia.

In July, 1859, M. Velpeau drew the attention of the French Academy of Sciences to the value of the mixture of coal-tar and sulphate of lime, of MM. Corne and Demeaux, in the healing of ulcers and other offensive wounds; and it may be added, that this mixture was used with great advantage in the French army, after the great battles of Magenta and Solferino.

In the following month I forwarded a note to the French Academy, pointing out that from experiments I had made with the various substances existing in coaltar, it was highly probable that carbolic acid was the active agent of the coal-tar used by MM. Corne and Demeaux; and that much more certainty might be expected if that acid were substituted in their mixture; for the composition of coal-tar varies according to the nature of the coal, and the temperature employed in its preparation. I also suggested that it was probable that the powerful antiseptic properties of carbolic acid prevented the decomposition of the adjacent parts, and thus tended to restore the wounds to a healthy state, and to remove the cause of infection. Before quitting this part of the subject, I beg again to call attention to a fact which I have already published in one of my papers, namely, that the addition of two or three drops of this acid to a pint of freshly made urine, will preserve it from fermentation or any marked chemical change for several weeks.

CHEMICAL NEWS, Jan. 11, 1862.

I have also applied it lately to foot-rot, which annually carries off large numbers of sheep; and I have been given to understand that the remedies hitherto adopted in this disease have been only partially successful. I think that if my experiments are further confirmed, it will prove a great boon to the farmers of this country. This acid has also been applied by me, during the last twelve months, to the preservation of gelatine solutions and preparations, of size made with starch, flour, and similar substances, and of skins, hides, and other animal substances. In fact, its antiseptic powers are so great, that it is the most powerful preventive of putrefaction with which I am acquainted. It appears also to act strongly as an antiferment; for I have proved on an extensive commercial scale, that it prevents (as stated by me in a paper published in 1855) the conversion of tannin into gallic acid and sugar. It also arrests lactic fermentation. I am now engaged in a series of experiments to discover if that power extends to alcoholic, butyric, and acetic fermentations. I hope also to communicate to you shortly the results of my experiments on the protection of timber from dry rot.-Pharmaceutical Journal.

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Course of Six Lectures on Light' (adapted to a Juvenile Auditory), by JOHN TYNDALL, Esq., F.R.S., Professor of Natural Philosophy in the Royal Institution.

LECTURE I. (Dec. 26, 1861.)

NOTES TO THE LECTURE:

Light travels from the sun to us in 74 minutes; it would take a cannon Light moves through space at a speed of 192,000 miles in a secondball 15 years to perform the journey-An express train travelling night and day would require 3 weeks to go round the earth; light would engine-If the sun were blotted out, we should continue to see it for 7 perform the same journey in the interval between two puffs of the minutes after its extinction-If the nearest of the fixed stars were blotted out, we should continue to see it for 5 years after its extinction.

Light moves in straight lines, and therefore opaque bodies cast shadows-If the opaque body be a sphere, and if the light emanate divergent cone-If the luminous sphere be of any sensible magnitude, from a mere point, the shadow of the sphere is a sharply defined but less than the opaque one, the perfect shadow will be still a divergent cone, but it will be surrounded by an imperfect shadow called a opaque body, the perfect shadow will be a cylinder, and it will be surrounded by a penumbra-If the luminous body be a sphere larger than the opaque one, the shadow will be a convergent cone, surrounded is the character of the shadows of the earth and moon in space-The shadow of a body placed in the sunlight is never sharp, but fades

penumbra-If the luminous body be a sphere equal in size to the

by a penumbra-In consequence of the great size of the sun, this last

gradually away at the edges.

When light strikes an unpolished surface, it is scattered irregularly in all directions-When light strikes a polished surface it is regularly flected along the perpendicular-If it strike the surface obliquely, the

reflected-If it strike the surface along the perpendicular, it is redirect and reflected rays are equally inclined to the perpendicular -This is expressed by the law, that the angle of incidence is equal to the angle of reflection-(Note. The angle of incidence is that enclosed between the direct ray and the perpendicular; the angle of reflection is that enclosed between the reflected ray and the perpendicular)-If a mirror from which a ray is reflected be caused to rotate, the ray will also rotate, but it will turn round with exactly twice the speed of the mirror.

In the case of a plane mirror, if the rays before reflection be convergent, parallel, or divergent, they will be convergent, parallel, or diver gent after reflection. The relative directions of the rays are unchanged by reflection from a plane mirror-In the case of a concave mirror, if the rays before reflection are divergent, they are less divergent after reflection; if the rays before reflection are parallel, they are convergent after reflection; if they are convergent before reflection, they are more convergent afterwards-The point to which parallel rays converge after reflection is called the principal focus of the mirror-In 1 Reported verbatim by special permission,

NEWS

the case of a convex mirror, convergent rays are rendered less convergent, parallel rays are rendered divergent, and divergent rays are rendered more divergent by reflection-If the rays of a parallel beam reflected from a convex mirror be prolonged backwards. they will cut in a point behind the mirror, which point is called the principal focus of the mirro-In a concave mirror, then the principal focus is a real point in front of the mirror where the reflected rays actually cross each other; whereas in a convex mirror the reflected rays do not actually intersect; hence the focus of the convex mirror is called an imaginary focus, in opposition to the real focus of the concave mirror -The image in a plane mirror is as far behind it as the object is in front of it-The image in a plane mirror is a lateral inversion of the object. A compositor's type, or a sentence written backwards, may be read as common print or writing when reflected from a piece of looking-glass-You can see your whole person in a looking-glass of half your height-If two plane mirrors be parallel, a luminous object placed between them gives an infinite series of images on each side Two plane mirors, so placed as to inclose an angle, give a definite number of images of an object placed between them-this is the principle of the kaleidoscope.

The principal focus of a spherical mirror lies midway between the centre of the sphere of which the mirror is a portion and the surface of the mirror-The image of an object placed between the principal focus and the surface of a concave spherical mirror is erect and magnified, and is behind the mirror-The image of an object placed between the focus and the centre of the sphere of which the mirror is a part, is formed in front of the mirror and beyond the centre, it is inverted and magnified-The image of an object placed beyond the centre of the sphere to which the mirror belongs, is also formed in front of the mirror, between the focus and the centre; but here it is inverted and diminished. This is the case observed on looking at the concave surface of a polished silver spoon-The image observed in a convex mirror is upright and diminished: this is the case observed on looking at the back of a polished silver spoon,

I take some pleasure, ladies and gentlemen, in going to the Alps every summer. I like climbing the mountains partly on scientific accounts, and partly for the simple love of climbing; but, usually, before I try any dangerous point-any high and giddy peak-I require three or four days' discipline. I dare not trust myself, fresh from London, upon a very high and precipitous peak. Unfortunately for me, morally speaking, I am at present placed upon what to me is a dangerous moral peak, without those few days discipline which I find of so much importance to me in the Alps. I find myself placed in a very peculiar position. However, I will shorten my preface, and make it very brief in reference to this point, The leader the noble leader who has hitherto led us up the mountain of knowledge, and before whom I have been as a boy and a listener for the last eight years, has thought proper to lay aside his alpenstock and to rest himself for a time. I hope sincerely, for your sake and for mine, that he will soon again resume it. (Cheers.) I say it falls on me to stand in his shoes; and, believe me, ladies and gentlemen, it is no pleasure to me to do so; but I feel that I have nothing to do with pleasure under present circumstances, for I, and every boy who hears me, ought to place his duty before his pleasure; and my duty I shall seek to discharge with your assistance-with your help-throughout the course of these lectures, to the best of my ability, throwing myself upon you for the recption that they are to receive. Just one word I will repeat of what Mr. Faraday has often told you at the commencement of his Juvenile Lectures -that they are really to be juvenile lectures, and although people of mature years, who know more, perhaps, of these matters than I myself do, come here, they will kindly be content to stand spectators while my young friends and myself converse together upon the subject that forms the object of our present consideration,

That subject is light; and in order to make plain to you from the outset the means we shall make use of in these lectures, I must commence by saying to you that I must have a source of light. It would not answer for my purpose to take a candle, to take the light of a lamp, or to take the light of these gas jets that you see around you. In order to make these phenomena visible to you all I must have a very intense source of light-a source of light almost as intense as the sun itself. And here I have two conductors of most mysterious power about which you, my young friends, know not quite so much, but almost as much as I do. These two wires which are covered with

gutta-percha, are the channels of a power which is now latent in the yard down-stairs. That power will rush through these wires; (we have nothing to do with it farther.) It will give us the means of producing this miniature sun which we shall use in these experiments. Here at the end of these wires we have two pieces of coal, and if I now bring these two pieces of coal skilfully together, you see that I produce a very intense light. The electricity is flying across from point to point producing this intense light. Now in our experiments we shall have to make use of the instrument that you see here, The bits of coal attached to these wires are fastened to this instrument and thus I am saved the trouble of holding them in my hand, and am enabled to keep them at a regular distance asunder, for it is necessary that they should be kept at a certain distance asunder in order to produce this glorious sun-like light. Here again you see coal points exactly the same as those that I made use of in the former experiments; and by means of a screw 1 can bring them together, separate them, and there you have the light. Now this is the glorious light by which we are going to operate at present, by which we must endeavour to track the phenomena of light, and by which we shall, I trust, learn something more regarding this wonderful agency which we have all passed by perhaps unheedingly from our infancy. We shall, all of us, I hope, learn something more of this agency than we know now. For we are all learners here-all students, all boys in fact, at the feet of Nature, endeavouring to learn-endeavouring to decipher the vast and glorious book which she lays open before us; and I say that the wisest philosopher among us, Mr. Faraday included, is nothing more than a boy in the presence of that glorious book,

Well then, in order to enable me to show you the course of these rays of light, I have to be grateful to Narure for giving us a hazy, foggy day in London, and here, at the present time, in this room, I have no doubt that many of you will notice a certain dimness of the atmosphere. That dimness I hail as a boon, because it will enable me, I trust, to show you the track of the rays of light through the air; for if there were no obstacle to the rays of light in the air-no dust suspended, or nothing upon which the light could fall,-the light would be absolutely darkness; in order to make itself evident to our sense it must fall upon something, I have here a lamp, precisely the same as the one you saw previously, and I have placed it inside a little box in order to shade it, and I will cause the rays of light to issue from the lamp, and will show you the track of light through the air of the room. I first bring my coal points close together just as in the former instance, and now you see we have the track of our beam of light. I do not know whether all of you see what I mean, but if you look you will see what I want you to pay attention to-the track of the beams-the straight line which they describe through space (for light is propagated in straight lines), and in this way I trust to be able to make the presence of the rays visible to you, and to enable you to track them through the room, and to develope their laws in the most beautiful and interesting manner. So much, then, for the instrument which I intend to use in this Lecture.

You have thus seen that these beams of light going through the room pass in straight lines; and I have taken the liberty of placing a few notes on this subject before you, for the use of those who will be pleased to be philosophers enough to try to repeat some of those experiments afterwards-who will entertain a sufficient love of philosophy to try and imprint what they have here heard upon their minds, and I have not a doubt (I speak confidently, not on my own account, but in virtue of the very importance of the subject) that although five-sixths of the boys present may perhaps forget all that I say, some of you will remember all your lives long what is said here; and I do not know but that in speaking in

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Royal Institution of Great Britain.

my own humble way at this table, I am scattering seed which may blossom when you become men;-you may become philosophers like Sir Isaac Newton; you may become great men; you may be able to lay hold of these treasures of Nature, and convert them into stores of power for the good of your country. Unfortunately, we cannot do this at present, but I trust that the boys of this generation, who will be the men of another generation, will be able to do it. Well, then, with regard to this agency that we call light, I must direct your attention to one of the first statements in the notes that I have caused to be placed upon the seats of the Institution. I speak there of the enormous velocity of light. You see we can measure that velocity. Light runs over a distance of twenty miles in 1-10,000th part of a second. You see we measure a thing that travels so quickly as that. I will tell you how it was measured in the first instance by a very able man. You all know the glorious orb that shines so redly through the London smoke at the present time, our sun. Well, at a vast distance from the sun there is a great planet called Jupiter, which planet is weightier-is heavier-than all the rest of the planets put together. This ball (s) I take to represent our sun. This ball (j), the planet Jupiter. As you know very well, the earth travels round about the sun, describing what is called the earth's orbit. That dark ball (e), I will ask you to imagine as representing the earth. I have drawn a broad line round the sun; this line is to represent the path in which the earth circulates round. Now we

will suppose that the earth travels round and round the sun incessantly through ages and ages. Here upon this little ball we live, and there are astronomers with their telescopes pointed to this great planet Jupiter. They can with their little bits of telescopes accurately tell you the size of the planet, tell you the weight of it in pounds, how much it weighs. But more than that, they have discovered that round about this large planet four beautiful little moons circulate. Now I have taken a white ivory ball (p) to represent the first moon of Jupiter, -the one which is nearest to Jupiter.

I do not want you to remember the other three moons at all at present, only that which is nearest to Jupiter, and to fancy that little moon going round and round the planet incessantly. Well now, we will suppose the earth is here (at e) an observer on the earth's surface looks at Jupiter through his telescope, and sees this little moon (p); he observes it travelling along, crossing the face of Jupiter, going round until finally it comes here (to p') upon the edge of the vast shadow that this huge planet casts through space. It is like a little lamp shining before the astronomer up to a certain point, suddenly it plunges into that shadow and is no more seen; it crosses the shadow, and by-and-bye as the astronomer still gazes, he sees the little lamp emerging from the shadow of Jupiter here (at p") as a bright light; and thus it goes round and round, and the astronomer is enabled to calculate with the greatest certainty the precise moment when this little lamp ought to appear. It can be calculated to the fraction of a second when it ought to dip into Jupiter's shadow and quench itself, and when it ought to emerge from Jupiter's shadow, and re-light itself. Now imagine six months have passed by, and the earth has gone, gone, gone away to here (é); our astronomer, still keeping watch on Jupiter's moonand this is a beautiful discovery-now finds that this

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CHEMICAL NEWS, Jan. 11, 1862.

little lamp appears to be about fifteen minutes later in emerg. ing from the shadow than when he was there, (at e.) Why? Because it requires fifteen minutes to travel across the earth's orbit. It requires a time of fifteen minutes-that is, seven minutes and a-half, as I have stated in the list of memoranda, to travel from the sun to the earth. If the velocity of the light travelling from this point (e) of the orbit to the opposite point (é) of the earth's orbit requires fifteen minutes, it will require simply half that, that is, seven and a-half minutes, to travel from the sun to the earth. And this is the velocity of light; it requires seven and a-half minutes to travel from the sun to the earth; its velocity being in round numbers 200,000 miles a second, exactly, it is 192,000. I will not repeat all the statements here made to you in the memoranda. The human mind cannot conceive of this velocity: and, in order to enable you to form some vague notion of the speed, I have compared it with the velocity of an express train travelling night and day at the velocity of forty or fifty miles an hour, which would require three weeks to get round the earth; whereas, in the interval between two puffs of the engine, light would travel over the same space. So much then for the velocity of light.

I now pass on to the consideration of another portion of the subject, and that is, inasmuch as light travels in straight lines, (as we have already seen when passing through the room,) if you place an object in the path of a ray of light, the light will graze past the sides of the object and will cause the space behind it to be shady. We have thus shadows produced. And now let me speak to you in reference to these shadows. I have here a lamp on the table; I trust you have cast your eyes upon the memoranda where I speak of the influence of the size of a luminous body upon the shadow that it casts; for if light move in straight lines-the light from this lamp, for instance-if it start here from a point, the rays of light will go diverging from that point, and grazing the two sides of that weight, it will go on in a straight line, and all the space behind the weight will be, as I have said, a divergent cone of shade. But if it emanate, not from a point, but from an object of sensible magnitude, such, for instance, as the flame of a lamp, as I have here, then, I say, that the light does not cast what we call a sharp shadow. It casts a shadow, but that shadow is surrounded at its edges by a fringe, which is called a penumbra. Now, when I hold that stick pretty close to the screen there is a very slight penumbra,-you perhaps see none; but I see a very little fringe, and as I draw it towards the light you will see the fringe expand. There it is pretty clear. You see in the centre it is a perfect shadow, but round about that you see a dim washy rim, which is the penumbra of the shadow; and that is because the light has a certain magnitude that what is shadow to one part of the light is not shadow to another; and there is only one space in the centre that is really perfect shadow, shaded from all portions of the light. This central portion is what we call the perfect shadow, and this fringe surrounding it is what we call the penumbra. I will now lower that light, and give you an idea of the sharpness that we have with this splendid electric light, in which, to some extent, the light, is emanating from a point, or very nearly a point, at least; the electric light having scarcely any magnitude at all compared with the light of the lamp. [Holding a stick between the electric light and the screen.] Now you see no penumbra; you see perfectly clear and sharply defined edges, and if we bring it near it is the same-perfectly clear and sharp. Those who look at it closely may perhaps see that there is now a little fringe, because the light does not emanate absolutely from a point; but if it had no sensible magnitude, then it would be a perfectly clear and sharply defined shadow in all positions of the stick.

Now, in order to make this plainer to you, I will make

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