CHEMICAL NEWS,} Feb. 1, 1862. The Behaviour of Essential Oils to Iodine and Bromine. watch crystal five drops of the ethereal solution of bromine are added. In using this test, the evaporation of ether which takes place from the crystal absorbs so much heat that the mixture is kept cool, and no generation of surplus heat can be expected. All the articles used in this examination are of a volatile nature, and their evaporation is favoured by a constant change of air; it is, therefore, requisite to prevent this, and make the experiments in a place excluded from draughts. The employment of an ethereal solution of bromine suggested the use of iodine in a state of solution; its ethercal solution appeared to have many advantages over the concentrated spirituous tincture, owing to the solubility of oils in ether, by which property a more intimate connection and a quick and sure reaction would be promoted. I have employed a concentrated ethereal tincture of iodine, which was prepared by adding to officinal ether sufficient iodine to leave some of it behind undissolved. For the same quantity of essential oils as in the above-mentioned tests, three drops of the ethereal tincture were added, in a place protected against draughts of air. The drops of this ethereal solution were larger than was anticipated; but I am not prepared to say whether their size is to be ascribed to the presence of iodine, or chiefly to the peculiar form of the lip of the vial from which they were dropped. When testing with the ethereal solution of bromine, a peculiar phenomenon had been observed, consisting of a spreading out of the mixture, up the sides of the watch crystals. This was referred to the evaporation of the ethereal liquid, and no further notice was taken of it; but while experimenting with the ethereal tincture of iodine, and noticing the same behaviour, there was a marked difference observed in this "spreading" from the presence of some oils, and attention was directed to it. The "spreading" consists in the uniform working up on the side of the crystal, towards the circumference of a larger or smaller quantity of the mixture, with a flapping or wavy motion, in some instances up to the very edge of the vessel; and then returning to the bottom again by forming streams from the upper margin down. Some oils show but little of this spreading motion, but mix quietly with the ethereal liquid, being acted on by the iodine and exhaling the ether; others, again, while they are miscible with little disturb ance, subsequently commence to spread, gradually running over the edge of the vessel, leaving but little oil behind. Sometimes the commotion of the liquids is such as to resemble a brisk effervescence or the phenomenon of boiling. While I at first supposed it to be all owing to the evaporation of ether, I am now inclined to think that the composition of the various oils exerts its influence on this activity, while a neutral behaviour in this respect of other oils can scarcely be founded on any grounds except peculiarity in their composition. I have in this connection to draw attention to a phenomenon, which, it seems to me, may be referred to the same reason. I refer to the peculiar motion imparted by certain volatile oils to a solution of bichromate of potassa mixed with sulphuric acid, which was described by Dr. J. T. Plummer, of Richmond, Ind., in the American Journal of Pharmacy, vol. xxviii., p. 197. I regret that my attention had not been drawn to the difference in this behaviour at an earlier season, so as to allow me sufficient time to report all the experiments with the ethereal solutions, with a view of observing the particularities of the various oils in this respect. My 63 notes on this subject are, for the reasons above named, rather meagre; but I give them as they were put down at the time, reserving for future observations, to correct and enlarge them, if the matter should turn out to be really of interest and importance, in recognising the volatile oils. Upon the behaviour of the volatile oils towards iodine various classifications have been based. Tuchen classified them into fulminating oils, in oils which dissolve iodine completely, and such which dissolve it but imperfectly. Zeller, from his careful investigations, divided them into five classes, and each of them again in various subdivisions. They are as follows: I. Fulminating or decomposing with detonation, with much heat, and the generation of violet and yellowish red vapours (a.) Quick and violent fulmination, with mostly violet vapours. Oleum terebinthinæ, sabinæ, juneperi, macidis ; (b.) Brisk but less quick and violent fulmination, with principally yellowish red vapours. Oleum neroli, bergami, limonis, aurantii, lavandulæ, spicæ, origani, vulganis, petroselini, herbæ, copaivæ. II. Quiet and noiseless evolution of yellowish red or gray vapours, accompanied with a rise of temperature. (a.) Many yellowish red vapours, considerable rise of temperature. Oleum cardamomi, melissæ, majoranæ, asari Europe; (b.) Few yellowish red vapours, with perceptible heat. Oleum rosmarini, serpylli, hyssopi, anisi vulgaris; (c.) Few yellowish red vapours, little heat. Oleum thymi vulgaris, salviæ, millefolii, cubebæ, cejeputi, menthæ crispa, matricariæ, arnicæ flor, anethi, foeniculi, anisi stellati, carui; (d.) Few greyish yellow vapours, little heat. Oleum calami, valerianæ ; (e.) Few gray vapours, little heat. Oleum nigellæ, cumini. III. Solution without vapours, but with a rise of temperature. (a.) Considerable heat. Oleum cinnamomi Ceylon ; (b.) Little and very little heat. Oleum cascarilla, cydoniæ, absinthii, cinnamomi Chin, caryopylli. IV. Solution without vapours and heat; (a.) Forming a homogeneous solution. Oleum cynæ, tanaceti, mentha piper, origani cretici, sassafras, rutæ, arnica radicis, petroselini seminis, sinapis; (b.) Forming two strata. Oleum asphalti, ceræ, succini. V. Partial and very sparing solution without reaction. Oleum amygdala amræ, rosa, petræ. The description of the colour as it is affected under the influence of iodine, is a matter of some difficulty; it occurs very often that in the first or a subsequent stage of the reaction, a colour is produced which strongly reminds one of the colour of an iodine solution, but at the same time so different as to be easily noticed. From the similarity of appearance it was suggested to compare them with the colour of a solution of iodine, and wherever in the following observations, the expression "iodine colour" has been used, it is understood to apply to a colour resembling that of tincture of iodine, diluted with alcohol to about four times its volume. From this explanation, other expressions, such as pale, deep, yellowish, reddish, &c., iodine colour, will be readily understood. I have to remark yet, that the oils which I have used in these experiments, were mostly obtained from reliable sources; in some instances the commercial articles were used, in which case I took pains to examine them first in other ways, so as to be assured of their freedom from adulterations. The physical properties of each of the oils examined by me will be briefly stated. In proceeding to state the reactions as they belong to the various oils, I thought proper to give also the observations of Zeller, which in all cases will be distinguished by an affixed Z. REACTIONS OF THE VOLATILE OILS WITH BROMINE AND IODINE. I. The Carbohydrogens.—Oleum Copaiba. Thin, colourless. Iodine. Faint fulmination, with yellowish red vapours and considerable heat, developed on stirring. After the reaction the residue consists of a reddish brown iodine compound, adhering like resin to the glass, and a syrupy liquid of a brown colour, changing into greenish by exposure to the air. Z. The reaction is not violent; the sediment has a blackish brown colour with a margin of yellowish brown, not iodine colour, the oil is yellowish, the whole miscible without separating. Ether sol. iodine. Some spreading, mixes with little sediment and a brown yellow colour; after six hours, greenish brown, little thickened, scarcely any sediment. Ether sol. bromine.-The reaction is accompanied by white vapours and a green colour; after the reaction the sediment is brown, the oil brownish green. Oleum Cubebæ. Oily consistence, limpid, a faint greenish tinge. Iodine.-Little heat, radiating motion, few reddish vapours; at first the oil is violet; after mixing, the colour is changed to yellowish brown, and the residue has the consistence of honey, and a somewhat - modified odour. Z. gray and Brisk reaction, without fulmination; yellow vapours: the colour is blue, afterwards bluish green; after the reaction the sediment has a nearly black colour, with a bluish green margin, around which the oil is of a lemon colour, growing fainter towards the edge. The whole is miscible to a dark greenish liquid, from which the sediment subsides again, leaving the oil of a greenish lemon colour. Ether sol. iodine.-Little spreading. The iodine colour quickly changes to yellowish and greenish brown, then greenish black. After six hours the oil is considerably thicker and of a black colour, which in very thin layers appears blackish green. Ether sol. bromine produces white vapours and a violet colour, growing deeper in a short time; after the reaction the sediment is of a deep violet, almost black; the supernatant oil of a dark greenish blue. Oleum juniperi baccæ.-Thin, limpid. Iodine. The oil fulminates quickly, with evolution of violet vapours and much heat, particularly on stirring. The residue is an oleoresinous, blackish brown mass, and some scarcely coloured oil, the whole miscible with some difficulty to a greenish brown, afterwards olive green liquid. The oil from unripe berries fulminates more violently and gives out with the iodine vapours, also many of a gray colour. With old oil the reaction is of shorter duration and shows less heat, the residue is easily miscible, and has then a reddish yellow-brown colour, and the consistence of an extract. In all cases the residue has a balsamic little modified odour. Z. By the reaction much heat is produced and many gray, but few violet vapours; the residue consists of a dark brown resin and an olive green liquid, which after a while are miscible to a half syrupy greenish redbrown liquid; the odour is scarcely modified. Ether sol. iodine.-Some effervescence and spreading; the oil assumes a greenish yellow colour, while an iodine coloured resin-like mass collects on the bottom, which after mixing separates again; the oil has now a light iodine colour, afterwards a brownish olive-green. Ether sol. bromine mixes with the oil with a radiating motion; the colour is brownish yellow, with streaks of brown, which turn to a brownish purple, afterwards purplish black; the supernatant oil is now olive green. Bromine produces a very violent detonation and many gray vapours; the residue by one drop of bromine is thin, pale yellow; by two drops, oily, dark olive green; by four drops, thick syrupy reddish brown; the odour of the first is nearly unaltered, of the others more modified, but plainly like juniper. (To be continued.) PHYSICAL SCIENCE. On Platinum Standard Kilogrammes. AT a late meeting of the French Académie des Sciences, M. Regnault presented a copy of the report on the comparison made in Paris in 1859 and 1860 of numerous kilogrammes in platinum and brass with the platinum standard kilogramme in the Imperial Archives. This report is printed at Berlin : "The Prussian Government possessed a platinum kilogramme, which on the 24th of October, 1817, was compared by Arago and H. de Humboldt with the standard platinum kilogramme in the Archives of Paris. The Austrian Government, on its part, had a platinum kilogramme made at Paris, which on August 20, 1857, was compared by MM. Silbermann and Froment, in the presence of M. Tresca, Sub-Director of the Conservatoire Impérial des Arts et Métiers with the standard platinum kilogramme in the Archives. A comparison instituted between the two platinum kilogrammes of Berlin and Vienna showed that they differed considerably. One of them at least must have been faulty. The only way to elucidate the question was to again compare these kilogrammes with the platinum kilogramme in the Imperial Archives of Paris. At the request of the Prussian Government, the Minister of Public Instruction_named a Commission, composed of MM. Regnault, Member and President of the Institute; Le Verrier, Member of the Institute and Director of the Imperial Observatory; Morin, Member of the Institute and Director of the Conservatoire des Arts et Métiers. On the other hand, the Prussian Minister of Commerce and of Public Works named M. Brix, Conseiller Intime and Director of the Central Commission of Weights and Measures of Berlin, to co-operate with the French Commissioners in again comparing the Prussian kilogramme of 1817 with the standard platinum of the Imperial Archives. M. Le Verrier was prevented by his occupations from performing his share of the work. The direction of the experiments rested then with MM. Regnault, Morin, and Brix. But we ought to mention the active and intelligent aid given by M. Silbermann, M. Tyarn, and by M. Deleuil, philosophical instrument maker. If the Commission had confined itself to re-comparing the Berlin kilogramme with the standard one of the Archives, and making the necessary correction, its work would have been less long; but on this occasion the Commission proposed to itself a longer and more difficult problem: CHEMICAL NEWS Feb. 1, 1862. Royal Institution of Great Britain. It proposed to study carefully all the circumstances We will pass on to the definitive conclusion of the report: PROCEEDINGS OF SOCIETIES. 65 ROYAL INSTITUTION OF GREAT BRITAIN. Course of Six Lectures on Light' (adapted to a Juvenile LECTURE IV. (Jan. 2, 1862.) NOTES TO THE LECTURE: The white light of the sun is made up of an infinite number of rays of different refrangibilities-Each particular refrangibility corresponds to a particular colour; hence the number of colours involved in solar seven, which are called primary colours. These are red, orange, yellow, green, blue, indigo, violet-Of these colours the red is the least refrangible, and the violet the most refrangible; the other colours being intermediate between these two-The solar beam is resolved into these colours by passing it through a prism: the coloured image thus formed is called the solar spectrum-The colours of the spectrum, when suitably blended, produce white light-A colourless image of the coal points o the electric light may be built up from the colours of its spectrum. "Our principal object was to compare the two new platinum kilogrammes verified at Paris with the platinum light is infinite-But for convenience sake wo divide these colours into standard in the Archives. We have weighed them in atmospheric air, and for adequate reasons not in a We have before shown the exactness of the corrections we made for the displaced air. Thus, for the Berlin platinum kilogramme No. 1 we have obtained the following results: vacuum. "We can confidently state, then, that in a vacuum the Berlin platinum kilogramme No. 1 is too light by 0-16 mgr.; and the Berlin platinum kilogramme No. 2 is too light by 1.85 mgr." We will now enumerate some of the most important results obtained by the Commission. The platinum kilogramme of Berlin verified by Arago and de Humboldt in 1817 was too light by 12.020 millimètres. The brass kilogramme of the College of France, too heavy by 3846 millimètres, has not sensibly changed in weight during ten years' exposure to the air without covering in the glass case of a balance. A similar platinum kilogramme, after having been several times in vacuo, where it often remained for several days, did not show the slightest alteration in the weighings, nor when weighed in the air. When a platinum kilogramme is placed in each pan of a balance, and successively weighed in air more and more rarified, the changes which take place in the apparent weight correspond exactly with the weight In other words, an of air displaced in each case. abnormal condensation of the air on the surface of the two platinum kilogrammes does not produce a sensible change in their relative weights. If there exist a condensation of this nature, it produces the same effect on each kilogramme. The condensation of air on the surface of a platinum kilogramme is in no case sufficient to alter sensibly the apparent weight. Two brass kilogrammes have the same relation of weight, whether they are weighed in vacuo or in the air, and the apparent weight is corrected by the weight of air displaced, calculated by the ordinary principles of physics. Under the conditions in which the Commission operated, glass produced no abnormal condensation of air nor humidity on its surface. The surfaces of platinum plates did not condense a quantity of hydrogen appreciable by the most sensitive balance; or, not to go farther than is warranted by experience, the condensation, if it exist, is exactly the same under a pressure of 64 millimètres as under the ordinary pressure of the atmosphere. Some substances have the power of drawing the colours more widely apart than others; glass, for example, does this more effectually than water, and bisulphide of carbon more effectually than glass-The drawing asunder of the colours by a prism is called dispersion; thus, the greater the distance between the red and violet ends of the spectrum, the greater is the dispersion. When sunlight falls upon a body, a portion of white light is reflected from the surface of the body-A second portion is reflected after it has entered the body to a greater or less depth-It is this latter portion which gives the body its colour-Different bodies have the power of absorbing, or quenching within them, different kinds of light-A red body is red because it has the power of quenching all rays except those which compose its red. A blue body is blue because it has the power of quenching all rays except those which compose its blueBrilliant crimson feathers, for example, if illuminated by pure blue light, are as black as those of a raven;-conversely, a pure blue would be black if illuminated by red light-When the human face is illuminated by a flame which contains no red rays, the lips and cheeks lose all their redness; in the same light red and crimson flowers lose their bloom-In fact, the colours of bodies are entirely due to the light which falls upon them. If the white light of the sun were simple instead of compound, we should have only light and shade in the world; but we should have no colour. A metal heated to whiteness gives a continuous spectrum as long as the metal remains in the solid or liquid condition-But when a metal has been reduced to vapour, and when that vapour is rendered luminous by intense heat, the spectrum of the vapour is usually composed of brilliant bands-Every metal has its own distinct system of bandsWhen metals are mixed together so as to form alloys, the bands of each metal are produced in the spectrum of the alloy -Thus, knowing the bands that each separate metal produces, we can determine from A luminous vapour absorbs those rays which it can itself emit With regard to the duration of the impression upon the retina, there is a little experiment which all my pupils present can make for themselves, and which is a very pretty one. It consists simply in taking a knitting needle, sticking a little silvered bead on the top of it by means of marine glue or sealing wax, and fastening the other end of it firm, and then striking it so as to cause it to vibrate. When you allow the sunlight, or even the light of a lamp or candle, to fall upon the bead, you see upon striking it, the needle vibrates, and the bead performs certain excursions, and goes on describing the most beautiful figures. This I say is an experiment that each can make for himself, it is extremely pretty and well worthy of your attention. Now, let me show you what may be done in this way. We have here such a piece of vibrating rod, it is not exactly a knitting needle, but something stronger and better made. I will illuminate this bead by a beam from the electric lamp and project an image of it upon the screen. You see the little spot of light upon the screen; I will now bring that quite to a point, and if I cause this to vibrate, you will see what beautiful figures I get. Here you see we get a curious figure of 8, which denotes the kind of vibration of which this rod is capable. You can also get figures far more beautiful than that. If I touch it with the fiddle-bow, I shall produce a figure of 8 with a fine crimped outline. Now instead of this vibrating bead I will take simply a knitting needle, and introduce it into the beam of light as before. We will alter our lens so as to bring it in focus to a little point, and cause it to shine brightly upon the screen. Then I touch that, and you have a beautiful circle of light. See what a lovely figure you have when I touch this knitting needle with the fiddle-bow. These experiments, and numberless others, you can all make for yourselves, and they are really well worthy of your attention. Now, a number of interesting and ingenious toys depend for their beauty upon this principle of the persistence of image upon the retina of the eye and among the number is the chromatrope. I will just show you one that will suffice. Here we have a specimen, and we will throw our fine disc of almost solar light in intensity, upon the screen, and then bring the chromotrope to a perfect focus, so that the definition may be perfectly sharp. If I now turn the winch, you see the curious appearance of motion produced, and if I turn it the other way the motion you see appears to take place in the other direction-all these movements depending for their effect upon the power of the eye to retain an impression for a certain definite period of time. illuminated will appear perceptibly thicker than the portion not illuminated. This explains the effect I have spoken of, viz. that the farther you go away from the object the greater is the amount of irradiation. Hence it is that, looking at the new moon, which is a body 240,000 miles from the earth, you see it encircles the dusky portion of the moon and appears to belong to a larger sphere altogether. This effect can be illustrated in another way. You see this fine platinum wire stretched from one end of this instrument the other. When I connect it with the galvanic battery, the mysterious power which rushes through it from the battery down stairs will heat the wire to a very high degree of incandescence, and make it white with heat; and then, I think, those at a distance will observe that the wire appears to augment in thickness, that is, its apparent thickness will be much greater than at present. The wire being now heated, appears to me to be much thicker than it was formerly, and I dare say to those at a distance the difference will be more apparent. If I brighten that wire, you will find it will appear still thicker. I can do so by sending a more powerful current through the wire. This I shall be enabled to do by shortening the wire, and thus lessening the resistance which it opposes to the passage of the current. You see how intensely it is illuminated now, and I have no doubt it appears thicker than in the former experiment. I think the wire will bear yet a little more heating. I will shorten it a little more, and then I will diminish the light of the wire by interposing a coloured glass. [In this experiment the heat was so intense that the wire fused and parted.] The heat has fused my wire, but I will try again. I want to show you the effect of darkening by cutting off a portion of the light of this wire. I want next to draw your attention to another subject mentioned in our last programme (for I wish to make clean work of our programmes, and not desert the memoranda until we completely finish them). I mentioned there a peculiar effect upon the eye which I have called irradiation. It is a learned word, but it is very well that you should understand such, because you will frequently meet with these terms when you read, as I trust you will read, books on Natural Philosophy. You know that when I look at any of you, as we explained in our last lecture, there is an image painted on the back of my eye. The friend whom I see before me is, at the present time, depicted with perfect distinctness at the back of my eye, turned, with his head downwards. At the present time he is moderately illuminated; but suppose, instead of the mild light upon a little boy's face illuminated by the lamps that light this theatre-suppose him intensely illuminated -suppose his face to shine with the brilliancy of the sun, or very nearly so, then, in virtue of the intensity of the light, the image of his face on my retina would appear a little too large and would encroach upon the circumjacent space, and I should see his face larger than it really is. I should also see it so much the larger the farther I went away from it. I have here drawn two rings, and I would ask you to direct your attention at the present time to them. The black line is exactly the same thickness as the white I think those at the distance if they were asked which of these lines were the thickest, even with the present illumination, would be inclined to give the preference to So much, then, for this question of irradiation. Now the white lines. But, in reality, the one is not a bit thicker we come to the main subject of the day's lecture, and I than the other. I will ask Mr. Anderson to hold the dia- will go on building up my argument by facts. I will program in the light, and I think you will see that the white ject, first of all, a slice of light from the lamp, upon the line is apparently the thicker of the two. Those at a screen, by means of this lens. Now, would you suspect distance will see the effect of this more clearly than those that there was within that image-white, and beautiful near at hand; and if Mr. Anderson now partly withdraws and colourless as you see it there-would you suspect that, that white ring from the light and lets one side be illumi- in that white image, you would have the most splendid nated and the other not, I think you will be able to perceive red, the most vivid orange, the most burning yellow, I the difference in the thickness of the ring itself; the portion I was going to say,-and green, blue, indigo, and violet,— one. You will have the wire bright as before until I interpose this piece of coloured glass. Look through the glass and at the wire, and compare that portion of the wire which has the light cut off with the rest, and you will see the difference. If I were to take a still darker piece of glass, the wire would appear much thinner, exactly as the size of the moon, looked at through a dark glass, on the Alps, becomes apparently less. CHEMICAL NEWS, Feb. 1, 1862. Royal Institution of Great Britain. all mixed up in that space of white. So that that light consists, not of a simple thing, but of the blending of all these colours-colours so intense, so beautiful, that no painter could imitate them. I will now take a little prism of glass and refract this beam of light upwards; you now see the image is refracted, and goes up as you remember it did in our former experiments. But it you look closely at that image you will notice a little colour-a little red at the bottom, and blue at the top. Now, I will take a larger and more powerful prism, and will cause the light to pass through it, so that it will be refracted upwards, and I think this powerful prism will be sufficient to cast the beam high up upon the wall, and you will then find this white light reduced to its coloured components. This is the grand discovery of the great Newton-for it was Sir Isaac Newton who discovered that the white light that you see is produced by the blending together of these splendid and beautiful colours. I want now to carry you through the proofs and arguments and considerations which depend upon this point. First, I will try and produce the self-same coloured image, or spectrum as it is called, by deflecting a ray of light sideways instead of upwards. I will place my slit vertically, thus, and if I make my coal points touch, a beam of light will pass through. I interpose my lens, and place the prism behind the lens and turn it so that the beam of light shall strike the prism. There, you have this beautiful image upon the screen-perfectly sharp and brilliant. I will now see whether I cannot actually squeeze those colours together, and mix them up so as to destroy that coloured image that you see. I place a lens behind the prism and I will see whether or not by means of this lens I can actually recompose the light-reblend it-remix it so as to produce the original white image. There, you see the light is so reblended that you have the various colours in a great measure destroyed. Supposing, now, I cut off with a screen the red which is on this side, what do I leave? You see the blue coming in on the other side; and so you see when I take a prism of this kind I can actually separate one image from the other. Now, I will try to deflect the red beam away. I can do it by partly interposing this small prism which you have had already. Here, then, you see the red is pulled away and now stands beside the blue; and again, if I deflect the blue and throw it to the other side, you see the red start out. These two colours which I thus separate when blended together produce white light. They are called the complementary colours. Now, let me see whether it is not possible, from this coloured image, to actually take the light of which it is composed, and to rebuild from it the very coal points from which the light issues. Let me see whether I cannot do that. These rays of light are, so to speak, the rough bricks and mortar of which the coal points are built. Here, then we have the light which composed the image, and now we have recomposed the coal points from which it issued. Let me now make an experiment of Newton's, for you to see the way in which he satisfied himself that the blending of these colours produced white light. He took a circular piece of paper, or paste-board, and he divided this circle into various parts, and coloured those various parts with the colours of the spectrum-one part red, another blue, another orange, and so forth, and then he set in it motion. He knew very well that by setting it in motion, because every colour remained on the retina during the time of the revolution of the circle, he actually threw all the colours together into the eye, and re-blended them. And in that way, when he set the card in motion, he produced a white disc. Now I have here such a circle, not as Newton made it, but of glass painted with transparent colours, and I project the image of this upon the screen, when you see that by turning this handle the colours completely vanish, because all 67 the colours are thrown simultaneously into the eye, and, being blended there, produce the impression of white light. Now this must satisfy you that the white light that comes to us from the electric light, and also from the sun, is composed of rays of a variety of colours. Some of these rays are more capable of being bent than others, and that is the reason why we are able to separate them. The blue is more so than the red, and the consequence is that, when we send them both through the prism, the one is separated from the other. I will now make an experiment with a prism of another kind. We have here a hollow glass prism with the sides cut and polished, and in this vessel is placed the liquid, bisulphide of carbon, that you see mentioned in the memoranda of this day's Lecture. This gives me a more richly-coloured spectrum than the glass. Now I will place this bisulphide of carbon prism on the stand. The light will come through this lens, and strike upon the surface of the prism; and it will pull the blue aside more than the red, and thus will separate the colours better. Here, you see, the colours of the spectrum are richer than those produced in my last experiment. Now, bear in mind that the beam of light, before refraction, would go on in a perfectly straight line. It is pulled aside by the prism, and you see the blue is pulled more aside than the red, and is, therefore, said to be the more refrangible colour, and the red is the less refrangible colour. Newton divided this bluish portion of the spectrum into three kinds; he called the first, that is the portion next the green, blue; the next indigo, and the extremity of the spectrum he called violet. Thus we have the seven primary colours: red, orange, yellow, green, blue, indigo and violet. I want now still more to augment the dispersion-remember that word; it means the pulling aside the colours of the beam. The amount of dispersion is expressed by the length, from the extreme red to the extreme violet: the greater its length, the greater is said to be the dispersion. Having now sent a beam through one prism, I will catch that coloured beam as it issues from this prism, and send it through another, and thus I can pull it round stil further. In that way I expect to get a spectrum that will stretch almost across the entire screen. I can twist that beam round so as to bring it upon the screen with a greatly augmented dispersion. There you see you have this glorious exhibition of colour actually stretched almost across the entire screen. Now you would call no portion of this image white, the paper has actually become coloured in virtue of the light falling upon it-the light has painted it. In point of fact all the colours that we see in Nature are produced in this way. Colours are not inherent in the bodies themselves at all. It is the light which falls upon them-which is in fact the paint-which gives beauty to every colour. However, I shall develope this subject more clearly as we go along. Now here at this end you see we have an intense beam of red light. I will let it fall upon a red object, and you will see that the object will appear red in it. I will show you this bunch of artificial flowers which are red, with green leaves: when I allow the red light to fall upon these flowers I think you will see that they are red, and you saw that they were red when the white light of the gas fell upor. them. Now this is a very important point which I wish you to perfectly understand. Why are these things red when the white light of the gas falls upon them? Simply because the cloth, or whatever it may be, or the colouring matter used in its manufacture-the dye of the cloth-has the power of completely quenching, absorbing, drinking in, destroying the blue, green, and yellow-all these are destroyed, and the only light which it is cable of sending back is the red what I say be if I pass these flowers into a coloured light, powering itself red, |
