Acetic acid Nitric acid Phosphoric acid Tannin Complete. Nearly complete. Ditto ditto. nitrogen existing in cast iron can be easily and speedily ascertained. Silicium.—I have ascertained by direct experiments Action less complete than above. that it is silicium, and not silica, that enters into the No similar action. Oxalic acid Tartaric acid Carbonic acid Gallic acid Oleic acid, Acid peat-water The action of acetic acid on grey cast iron is most interesting, for instead of ceasing when saturated with oxide of iron, as is the case with other acids, its action is continuous if the precaution is taken to close the mouth of the vessel with an ordinary cork. Thus I have had cubes of cast iron in contact with the same quantity of acetic acid for two years, and the chemical action still existed when the contents of the bottles were examined. This action of acetic acid appears, therefore, to be analogous to that which it has on lead. To examine the chemical composition of the cubes transformed by the action of acetic acid, they were reduced to fine powder in an agate mortar, and well washed with boiled water slightly acidulated with acetic acid. The powder was then dried at 115° C., in a dry atmosphere of carbonic acid or hydrogen, according to the nature of the body to be determined in the mass, The carbonaceous substance so prepared presented the following properties and composition: The cubes of grey cast iron, which originally weighed 15°324 grammes, weighed only 3'482 at the end of the two years, and their specific gravity was reduced from 7.858 to 2.751. Their composition was as follows: Composition of the carbonaceous substance. 79'960 11.020 • 95'413 These results led to the following remarks.Nitrogen. That the largest part of the nitrogen originally existing in the cast iron remains in the graphitoid substance, and only a small portion is trans: formed into ammonia, These facts tend to prove that the nitrogen of cast iron exists in it under two states, namely, that one portion is combined with the carbon, whilst the other is in a condition to unite with the hydrogen liberated from the water, and thus to form ammonia. This method of ascertaining the amount of nitrogen in cast iron, by determining the quantity of nitrogen in the state of ammonia, and that existing in the carbonaceous mass, appears to be the very best process known, for I obtained 0.790 from the same cast iron which only yielded me o100 by the process lately published by M. Fremy. For ordinary analyses of cast iron, the slow action of acetic acid may be advantageously replaced by that of hydrochloric acid, taking cale to use freshly distilled acid, water free from ammonia, and placing the whole in a flask with cork and tube, so as to exclude the possibility of provided the ammonia existing in the atmosphere from interfering with the experiment. By this means the amount of composition of the carbonaceous mass. Amongst other experiments I may state that I took 5'96 grains of the carbonaceous substance dried at 115o C. in a current of carbonic acid, and after having placed it in a small platinum dish, the whole was introduced into a porcelain tube, and submitted for several hours at a red heat to a current of pure and dry oxygen. I then found that the 5.96 grains had increased to 7.39 grains, or nearly the theoretical amount: for 596 grains of substance would lose 0.664 of carbon and the remaining 0.338 of silicium would become o'718 of silica, whilst the 4.672 of iron would become 6.702 of peroxide of iron, or both added together 7420, being within o13 grain of the weight actually found. Now, if the iron in the mass had been in the state of protoxide, and the silicium in the state of silica, the 5'96 grains employed would, after the loss of carbon, have decreased to 5'551 grains, leaving, therefore, no doubt that the carbonaceous substance contained metallic iron and silicium. Finding, however, that the quantity of silicium in the carbonaceous substance, though high, did not represent the whole of the silicium contained in the cast iron employed, I passed the hydrogen, liberated by the action of weak acids on cast iron, through fuming nitric acid, and found, on the evaporation of this acid, a white deposit of silica. All acids which give off hydrogen when in contact with cast iron, also give rise to the gas discovered by Wöhler, namely, silicide of hydrogen. Carbon.-Like silicium, the quantity of carbon found in the carbonaceous compound does not represent the whole of the carbon pre-existing in the cast iron employed, as carburetted hydrogens are given off during the slow action of acetic acid on cast iron,1 Iron. As shown by the above analysis, the carbonaceous compound contains 79'960 of metallic iron, even when the acetic acid has ceased to act upon it. I have made several experiments to satisfy my mind that the carbonaceous mass contains metallic iron, and not oxide of iron. Thus, I passed hydrogen, at a dull-red heat, over some of the carbonaceous substance previously dried at 115o, and obtained no water, and the experiment related under the head of silicium confirms this conrelative proportions of carbon and iron; but as cast iron clusion. All grey cast irons appear to yield the same becomes harder and whiter, the amount of carbon decreases, and in fact, nearly disappears in Welsh white cinder iron, being replaced by silicium. The relative amount of carbon and iron in the carbonaceous substance corresponds to 4C and 6Fe, or are the same that I have found in some cast iron which had been saturated with carbon, by melting No. I cast iron in presence of a large excess of coke on a cupola, and called technically "keechy." stance obtained by me is simply composed of 4C and I do not, however, believe that the carbonaceous sublikewise enter into its composition. But, in the present 6Fe, but that the nitrogen and silicium found in it must state of my researches, it would be premature to attempt I am, however, still pursuing the subject, I shall be to assign any definite composition to this substance. As happy, if I arrive at any conclusion, to communicate the same to the Society, hydrocarbons to enable me to submit them to a careful examination. I am now ongaged in preparing a sufficient quantity of thes CHEMICAL NEWS Feb. 15, 1862. Royal Institution of Great Britain. When the graphitoid substance prepared by the above process is exposed to the atmosphere, it absorbs oxygen with rapidity, and the temperature of the mass rises rapidly, protoxide of iron being first formed, which is converted into sesqui-oxide; but when this mass is placed in distilled water, a chemical action ensues similar to that described by M. Kuhlmann, namely, a portion of the carbon is converted into carbonic acid by the oxygen of the sesqui-oxide of iron, and the carbonic acid thus produced unites with protoxide of iron to form carbonate of protoxide of iron. The atmospheric action on the carbonaceous substance above described, explains the difference of composition which I have found to exist between the body obtained by me, and that of a soft graphitoid mass which was found to replace a mass of iron buried for many years amongst coal cinders, and which had the following composition: Mean of Several Analyses. Carbon Silica Sulphur 66.61 12.03 18.13 0'79 traces 2.14 99'70 89 process for preparing Schweitzer's liquid, he determined that, in this particular case, the solution of the copper was not due to atmospheric carbonic acid, but to the spontaneous production of nitrite of copper, an important fact with reference to the natural formation of nitrates. It is this fact which M. Schönbein has just confirmed. The formation of nitrous acid is determined by the production of ozone. Other metals besides copper give a similar reaction.-Journal für Praktische Chemie. PROCEEDINGS OF SOCIETIES. ROYAL INSTITUTION OF GREAT BRITAIN. A Course of Six Lectures on Light' (adapted to a Juvenile LECTURE VI. (Jan. 7, 1862.) NOTES TO THE LECTURE: The colours of thin films are produced by interference-The light falling on the film is partially reflected at its front surface; but a portion of the light enters the film and is reflected at its back surface -If the film be of a suitable thickness, these two beams will clash and destroy each other-By a different thickness, the two beams may be made to combine and help each other-To cause two red rays to coincide requires a thicker film than that necessary for the coincidence of the blue rays-Hence, a thickness which destroys one colour developes another. If the thickness of the film be uniform, the colour will be uniform; if the thickness vary, the colour will vary. It is to the varying thickness of its film that the varying splendours of a soap bubble are due. Nitrogen was also present, but its amount was not determined. I am aware that the latter curious and interesting change in cast iron has been several times noticed. Thus, Dr. Henry published a short notice on this subject in the "Annals of Philosophy." Mr. R. Mallet also described a similar substance in the Transactions of the British Association. Mr. Warington has also noticed the conversion of cast iron into a soft black substance, which occurred in a brewery where it was in contact with sour beer. Lastly, Mr. E. W. Binney records, in the eleventh volume of the "Memoirs of the Manchester Philosophical Society," a description of a similar substance found by him at the emptying of a deep coal-pit in which the cast iron had remained sub-refraction-When, for example, the image of the coal points of the merged for a few years, and which yielded to Dr. Angus Smith the following composition A beam of ordinary light must be figured as performing its vibra tions in all directions-By reflection at a particular angle these vibrations are reduced to a common plane: a beam, with its vibrations thus circumstanced, is said to be polarized-Thus light reflected from a plate of glass at an angle of 35 degrees, is polarized-A portion of the beam which passes through the plate of glass is also polarizedBy adding a second plate of glass, a still further portion of the transmitted light is polarized; and by using a large number of plates, almost all the light they transmit may be polarized-On passing through certain crystals, a beam of light is split into two, one of which is more refracted than the other. This is called double electric lamp is on the screen, the interposition of a piece of Iceland spir produces two images instead of one-The two beams emergent from the spar are polarized at right angles to each other-Thus light is poluized in three ways; by reflection, by ordinary refraction, and by double refraction. A polarized beam has two sides: when one of these sides strikes upon a gla-s surface at the proper angle, the beam is reflected; but when the other side strikes the same glass surface, the beam is not reflected-Thus, preserving the angle of incidence, perfectly constant, and causing the mirror to move round the ray, in one position w● have a maximum reflection, in the other a minimum; the quantity of light reflected in intermediate positions being intermediate between those two. Albeam of light, on passing through a plate of tourmaline, cut parallel to the axis of the crystal, is polarized-This beam can pass through a second plate of tourmaline if its axis be parallel to that of the first-But when the axes are crossed at right angles, the beam which passes through the front plate is cut off by the one behind By the interference of the rays polarized by crystals, splendid colours are produced. I think I ought to warn you at the outset that this is going to be a very hard lecture, but you have been so good On the Action of Oxygen on Ammonia in Contact with and so attentive during the last five lectures, that I am Oxides, by M. SCHÖNBEIN. BERZELIUS has stated that what was regarded as a solution of oxide of copper in ammonia is, in truth, only a solution obtained by the aid of an ammoniacal salt; he has proved that pure oxide of copper is insoluble in pure ammonia, but that if but a single drop of an ammoniacal salt, carbonate, for instance, is added and the mixture stirred, a beautiful blue colour will appear, sometimes dark enough to make the liquid opaque. When M. Peligot published his simple and elegant quite persuaded I can calculate upon your attention 1 Reported verbatim by special permission. go along and strike against the side of the canal; the moment they strike the side of the canal you will find that they produce a series of waves crossing the original ones, and this reflection of the waves is exactly the same as we have found to be the case for light, the angle of incidence is equal to the angle of reflection; the reflected waves fall as much on one side of the perpendicular as the direct waves are on the other side of the perpendicular, and thus you see the whole surface of the canal chased, reduced to a beautiful mosaic owing to the reflection of the waves in this way. I have often stood with joy and pleasure upon the sea coast of the Isle of Wight, down at Ventnor, and while a storm has been raging, have watched the waves come in higher and higher, and I have seen them heel up against the shore obliquely, and instantly another wave was reflected back crossing the first, showing that the reflection of the wave was exactly the same as the reflection of light. In connection with this point, I want, in passing, just to make one experiment to show you the production of the beautiful figures that are formed by the reflection of waves. I might have taken mercury which gives a very pretty image because it is a particularly bright and beautiful metal, and reflects the light in intense quantity, but instead of that I intend to make a cheap experiment and simply to take water. I will put some water into this tray, and we will throw the beam of light upon the water so as to illuminate it, and the water in the tray will reflect the light upon the screen, and there I trust, if you are all perfectly still, (mind that is the condition, if you are not perfectly still you will shake the tray and produce waves when we do not want to produce them.) If you are, as I have said, perfectly still, you will see an image of the surface of the water quite clearly upon the screen, and then I will agitate the tray, and you will see the beauty of the waves that are produced. There is nothing more beautiful than these wave motions, and in order to get them in a fine manner, I have here, you see, a little glass tube drawn at the end to a fine point, and those who have very good eyes will see little drops of water falling from the point. I will cause these drops to fall upon the water in the tray, and then I shall obtain a series of waves and you will see what beautiful figures we shall have if I am skilful enough to make it aright; the water will do its duty if I do mine, of that I am perfectly persuaded. In the former lecture I made an experiment of this kind with a square tray, and you had thus square waves reflected from the straight sides of the vessel, but here the waves are oval. Look how they cross each other and produce this beautiful change. I drop the water on the centre and you see how the waves coil and encircle each other. Thus you see that science makes beauty cheap. I might go on for any length of time producing these figures. I will change the point of dropping, I will go along the diameter of this tray, and here you see the varying figures, the different shape of the figures depending upon the point on which the drop falls. See how beautifully these run into each other. This, as I have said, is a very cheap experiment, and shows beauty in a very simple form. Now I come to one of the difficulties, and here I want your attention. I spoke a little during the last lecture of sound, the sound of my voice for instance, and I made an experiment, which, I ani perfectly certain pleased you all, to show you that the sound passed from my voice through the air to a little flame, and caused that flame to produce a very beautiful note, to sing a most melodious song. There is a passage from the outer air into the brain, that passage is stopped at a certain place by a thin membrane which covers the chamber something like a drum, and hence this particular membrane and the chamber that it covers are called the "drum" of the ear. When I speak, vibrations of the air enter my ear and strike against this drum and put it in motion. I draw this fiddle bow against a tuning fork; at the present moment the drum of your tar would be seen ben ing in and out. So if you take two watches, one ticking nearly in the same time as the other, supposing one produced 100 ticks while the other produced 101; they start together, then the two ticks unite to give an additional impulse to the ear; at the 101st tick they also coincide, there you will have the two ticks going again and driving, as it were, the drum of the ear inwards, and pressing the drum of the ear outwards. As I have said, the drum of the ear is pressed inwards first, and then outwards by every sound you can conceive; I am sure the simplest of my philosophic hearers can understand the possibility of this, that one watch may be in the act of pushing the tympanum in (I have used a learned word, which I did not intend to do, this membrane is sometimes called the tympanic membrane) one watch may actually be in the act of pushing this membrane in when the other is in the act of pushing it out. And what would be the consequence? the one would neutralise the other, and by the union of both the sounds we should have silence. Take them separately and you produce a sound, take them both together, and in virtue of their action upon the drum of the ear the one would neutralise the other. Thus you have often heard the beats of organ pipes, I am sure you have many times, and these beats are simply due to the fact that at certain times the sounds of the pipes coincide and produce a loud sound, at certain other times they go in opposite directions and produce comparative silence. Here are two organ pipes, when I blow strongly into them you will find the sound goes up and down, and you have this effect that we call "beats." This is produced, as I have said, by combination,-at certain particular times the sounds coincide and produce a strong pushing of the membrane in, at other times one pushes it in and the other out, so that we have sound and silence alternately instead of a continuous sound. Let me try whether I can do it; of course my lungs are not sufficiently strong to make it a loud sound, but if I had a strong pair of bellows you would have the sound very loud. You hear the beating, and that is produced by what we call the interference of the sound. Now, just let me say one or two words more upon this point, and then I will have done with it. When this tuning fork, or any other sound-vibrating body, is caused to sound in air, it vibrates thus. First of all, when it comes out it crowds the air together,-it pushes the air together, when it goes in it pulls the air asunder again. What is the consequence? Why, that ir a line of sound the air is thus circumstanced. At first the particles are crowded closely together, and I draw the lines closely together; then by degrees you have them wider apart owing to this action we have been speaking of, and then again they are crowded together, again widely apart and so on. And thus we philosophers can actually look at those things that quite defeat the ordinary eye. We can see, and you can see I am sure, the air between this tuning fork and yourselves, in a state of condensation and expansion, just as I have represented on the board. But now imagine two sounds to start from different points,-just let me define one point, the distance from this point of condensation (a) to this one (b) on our diagram is sometimes called a wave, it is called the length of the wave, these are sound waves in the air. CHEMICAL NEWS, Feb. 15, 1862. Royal Institution of Great Britain. Now conceive that at one and the same moment this sound starts from here (a) another sound starts from here (a) so as to produce a condensation underneath this expansion. Then you see the one sound will actually blot out the other along the whole line, and thus we shall have a case of interference of sound. And this is always the case when one sound is half a wave length behind the other,—the one sound destroys the other. Now do not be daunted, but give me your patience for one or two minutes more, whilst I pass on to light. A similar thing occurs with light. Light has this peculiarity, that whilst all the rays of sound, the particles of air between me and you, or between this tuning fork and you-when they are thrown into motion,-oscillate backwards and forwards, to and fro, in this way all along the line. The particles of the substance causing the waves of light do not oscillate so, they oscillate transversely across the line of the ray. That is the difference between sound and light. Let me take the case of a very thin film of glass, or of a soap bubble, or of varnish such as Mr. De la Rue uses to illuminate his paper; supposing I allow the ray of light to fall upon that film, a portion is reflected at the first surface and a portion goes through and is reflected at the second surface, and if the thickness be suitable these two rays will interfere with each other just like the two sounds we have been speaking of and will blot each other out and the consequence is by a film of a certain thickness you can blot out the rays of light as we have supposed the rays of sound to be blotted out. One word more, the waves of light differ in length, and you will see in the notes of our last lecture that the waves of red are longer than the waves of violet; the consequence is that the thickness, the necessary difference in the part travelled over by these two rays is greater in the case of the red than in the case of the violet, so that when we have an extinction of violet we have the red in all its glory, and when we have an extinction of the red we have the violet in all its glory. And this is the reason why we see these beautiful colours in these thin films. I won't trouble you with more than this, it is as much as you can bear, and I would not presume upon you too much. A Let me go on to another portion of the subject. You see I have here a piece of black glass, and it has a reflecting surface. If I cause a ray of light, falling upon a glass, to be reflected from it towards the ceiling, it is very curious but that ray of light before it strikes that reflector is vibrating transversely in all directions, but when it strikes the black glass and is reflected up, all these vibrations are performed in one way, they are all reduced to one common direction. Have you ever seen a snake going along a flat surface? A snake would represent a polarized beam, it wriggles along on the flat surface and all its oscillations or vibrations as you may call them that describe sinuosity are confined to the flat surface. serpent, however, wriggling along with vertical folds would represent also a polarized beam. The snake's mode of progression is horizontal, and the serpent's is vertical. I want you to have a clear image in your minds, that this ray of light which is reflected from the surface, goes on Oscillating in one direction only. Well now, I will show you next, that if I obtain a ray of light and cause it to be reflected upwards, and try to reflect it with another reflector, I can reflect it when I allow this beam of light to fall thus, but if I allow the side of the ray (for a ray, as it were, has two different sides), if I allow the side of the ray to strike this one, I cannot reflect it at all; it is quenched. Let me show you this experiment, and that will be better than any description. Here I will place this reflector in front of the lamp, and we will get a clear image of the disc upon the ceiling. Now, I say, that that beam that thus goes up to the ceiling is totally different in quality from the beam coming from the lamp. Let me show you this. If I take this second reflector and hold it in this 91 direction, you see I can get the image clearly reflected upon the gallery. If I hold it so as to cross the ray, as you see at the present time, it is extinguished. So you see when the side of the ray strikes the reflector it is not reflected, but when its face side strikes the reflector it is reflected. This is called a polarized ray of light; the ray has two sides. When one of these strikes against the reflector it is reflected; when the other strikes it is not reflected, it is quenched. Now, let me pass on to another point. We have here two little crystals, and those crystals are among the most wonderful things in Nature; they are what we call Tourmaline crystals. I will show you these upon the screen. This is a crystal, I should say, about an inch in length, and less than a quarter of an inch in width. Now, these crystals possess this peculiarity; if a beam is polarized so that it vibrates horizontally, so that its vibrations are parallel, running along the axis of the crystals it can get through it; but if I hold the crystals vertically and let a beam of this kind fall upon it horizontally, it is completely cut off. I will show you this. If I hold two of them together with their axes in the same direction, the light from the lamp passes through both, but if I cross them and look at the lamp through, I find at the space where they cross, the light permitted to pass by the one is completely cut off by the other. Let me see whether it is not possible to be shown to you. There is the image of one of our crystals. I will bring it to a clear focus on the screen. I now introduce the other one so as to allow it to fall over the other, and there you see the crystals are transparent, but I will turn them cross ways, and look, the centre rays where they cross are cut off. I turn them so as to render them parallel, and the light goes through both. I turn them cross ways and they are black. Now, both of these rays, which we get through these Tourmalines, are polarized. When once the ray has passed through the Tourmalines, it is compelled to carry on its vibrations all in one way, and that is the meaning of the term "a polarized ray.' How now shall I deal with this subject? It is very difficult to render clear. Supposing I allow the light that has passed through these two Tourmalines to fall on a reflecting surface. If I had the Tourmalines crossed, what would be the effect? One allows the rays to vibrate in one direction, and the other in another. Then if I hold this reflector at the proper angle, I shall be able to reflect one of the Tourmalines so as to remain bright, whereas the light from the other Tourmalines, according to what I have said, will be completely quenched, because it cannot be reflected. I hope you can follow me, for it is not the beautiful things which you may see upon the screen, but it is the beautiful thoughts which underlie these things that ought to interest you and will interest you most by and bye: only have a little patience with me now. Mr. Anderson will hold those two Tourmalines in front of the lamp and I will subject our reasoning to a test. This is the beauty of natural philosophy, that a man cannot go on wandering in a labyrinth of error; if he is wrong he is soon checked by Nature, and that, as I have said, is the grandeur of philosophy in comparison with other modes of thought and research. I will allow the beam from the Tourmalines to fall thus upon the reflector, and will reflect it upon the ceiling; you will find one of them black and the other bright. According to what I have said it ought to be so, let us see whether it is not the case. I am happy to see that by an accident one of these is longer than the other. You can remark that one is long and narrow, and the other is wide. Will anyone tell me, will my goldenhaired friend in front tell me, which is the longest, the plack one or the bright one? A VOICE: The black. Professor TYNDALL: Quite right; now bear that in mind. I will turn the reflector so, and thus I will project the image upon the screen. You saw that formerly I allowed the one side of the beam to fall upon that mirror, now I will allow the other side of the beam to fall upon the mirror, and let me see whether we get the same result, which is the black, is it now the long one? A VOICE: No, the other is. Professor TYNDALL: The other, to be sure, and thus we can verify the conclusion we previously arrived at. I have now to make one or two more experiments upon a point of very great importance. I have here a piece of crystal-one of these wonderful edifices that are built up by Nature herself where she takes the atoms of matter and puts them together so as to build up forms of the most exquisite beauty; this, then, is one of those crystals found in Iceland, which, because they are particularly beautiful and transparent are called Iceland spar. Looking through that spar you see all things double, perfectly double. That spar, when a single ray of light falls upon it, has the power of splitting it into two,-dividing it into two equal beams, they are both refracted, but one is refracted more than the other, and the consequence is that you have a splitting of the ray into two rays. I have here a piece of such a crystal cut out from a thin transparent specimen of Iceland spar; I will send our beam through this crystal, and you will find we can in that way split the single beam into two, so that instead of one image on the screen you will have two, and I will try to show you the track of those two beams through the air of the room. Here I place my piece of spar, and see how beautifully they come up: just look at those two images. Take that spar away and you have a single image, put the spar in and you see what takes place. That wondrous substance has the power of splitting these beams into two. And if I turn this round so as to bring one over the other, many of you can see the track of these beams through the room. As I turn those round and round, I think many present will see two beautiful cylinders of light passing through the room, showing you the splitting of the ray into two parts. You see that immediately it emerges from the spar the splitting of the ray takes place. One peculiarity of those two rays that we get from the spar, is that they are totally different from ordinary light. Now, what have I to do next? I want to investigate those two NEWS beams-I want to examine whether those two beams that issue from the spar are like ordinary light? They are not; those beams are both polarized, the rays of one are oscillating horizontally, and the rays of the other are oscillating vertically. Now, I only wish I had you for a few hours after all these lectures, that I might make you all undergo an examination, but inasmuch as I cannot have an examination, I will ask you a question. I am sure, from what you have now learned you will be able to tell me, from an experiment I am going to make, the manner in which this invisible thing which causes light is vibrating. I think I may almost appeal to you, Sir, [addressing one of the little boys,] to decide this highly philosophical and important point. Well now, let me see how I am to attack it. I will first of all produce those two images. Now, supposing I put a plate of Tourmaline in front of the lamp, I shall have two images of that plate of Tourmaline on the screen-an image for each beam. But we know that a plate of Tourmaline allows only rays that oscillate in a certain direction to pass through it,-that if they go in the same direction as the axis they can get through, whereas, if they go perpendicularly to the axis they cannot get through; so that in these two images of the Tourmaline upon the screen you will see one bright, answering to the ray that oscillates along the axis, and the other black, answering to the ray that oscillates perpendicularly to the axis. Let us see whether we cannot do that. Now, I say that one of those beams-I do not know which-oscillates horizontally, and the other will oscillate vertically. I now place my Tourmaline in front of the lamp,-the one image is black, and the other is bright. Now, let each young gentleman give himself time to reflect, and I ask you first to fix your attention on that right-hand beam. May I ask you, Sir, in what directions the vibrations are moving: are they horizontal or vertical? A VOICE: Vertical. Professor TYNDALL: They are vertical,-they vibrate across the axis,—and thus they are intercepted, and the image is perfectly black. Now, let us go on with our reasoning. We will test it still further. I have here a reflector,-and I think, in order to enable you to distinguish one beam from the other, I will produce a little magic, a little necromancy: I will colour those two dises for you by that wonderful property of interference. I will throw a bit of transparent crystal into the path of those beams, and that will give you two beautifully-coloured discs instead of the white ones. Here is the cryst 1; you will find it competent to colour those two discs, and, what is more, to colour them differently. The two colours I am going to produce will be complementary colours, and you all know what complementary colours are, they are those which, when blended together, will produce white. If I turn this, you see I change my colour. How beautiful they are! The right hand colour is now yellow, and the other is blue;-these are complementary colours: they produce white, although if you mix Prussian blue with gamboge you produce green. I will cause these to come a little nearer to each other; I will cause them to be as much contrasted as possible. You see, as I move this round I get a succession of colours. Here are red and green, blue and yellow, green and red again,-all complementary colours. Well now, gentlemen, let me go on with my reasoning. You see in our reasonings we build up our argument, as it were, by those glorious facts; our logic, as it were, makes those facts upon which it hangs itself. It is not a mere heaving up of ideas, but it is really that we take hold of the glorious facts of Nature, and we follow them and trace their connexion, and we, as it were, get up to the very intelligence that built up Nature, the very intelligence that actually pronounced the fiat that endowed Nature with these glorious laws; it is our privilege, and we are entrusted with the exercise of that power to make ourselves acquainted with these things, and to see the intelligence, |