May 30, 1878] 



NATURE. 



125 



II. Coloured light analysed by a prism gives us a dis- 

 continuous spectrum. 



III. Light may be coloured because originally_ the 

 series of its components was not complete. 



IV. Light may be coloured because although the series 

 of its components was once complete, some parts of the 

 light have been absorbed in its passage to the eye. 



V. The bodies which give us white light are generally 

 complex as to their molecules. 



VI. The bodies which give rise to the phenomena of 

 bright or dark lines are generally simple as to their 

 molecules. 



I now begin, with fear and trembling, to touch upon a 

 part of my subject in which I ought to be the first to 

 acknowledge that our ideas are not of the sure and certain 

 kind. In what has gone before I have been careful to 

 point out that, though the effect of incandescent bodies 

 in producing and absorbing light was not likely to be 

 directly applied by the artist, it was still in this sure and 

 certain region that he should endeavour to follow the 

 workings of laws clearly made out which might be in 

 force elsewhere. 



This brings me to state that in my opinion the 

 colours of most natural bodies depend upon the fact 

 that there are definite molecular states of all kinds 

 of matter lying between those two extreme stages to 

 the phenomena presented by which attention has been 

 directed. Some years ago, in a communication to 

 the Royal Society, I drew attention to evidence which 

 seemed to indicate that many substances which emit 

 under certain conditions a white light giving a continuous 

 spectrum, and coloured light associated with the spec- 

 trum of lines under others, exist also in molecular group- 

 ings between these extreme conditions, giving us for one 

 grouping a continuous spectrum at the red end, and for 

 the other a continuous spectrum in the blue. 



This is the most general statement that I can make, 

 and I make it on account of the utility of such a state- 

 ment. It has not yet been proved to be universally true, 

 but the evidence I have already accumulated justifies 

 me in setting it up as a working hypothesis. Not least 

 valid among the lines of evidence on which I rely is the 

 curious fact that the colours of almost all natural bodies 

 can be at once explained by assuming them to be built 

 up of these two molecular groupings to which I have 

 called attention. 



Let us take gold as an instance. It is yellow ; but why 

 is it yellow ? Because the molecules of gold, as I believe, 

 generally exist in two complexities, one of them compe- 

 tent to harmonise with the red rays of light, and therefore 

 to absorb them, the other doing the same thing with the 

 blue light, and for the same reason. 



Gum a piece of gold leaf on a piece of glass for easy 

 manipulation, and look at a bright light through it ; it 

 will be seen that the gold is green, or, in other words, 

 that the blue and red have been absorbed, we have 



W © © Y @ ^ 



changed into 



V I B©YOR 



by tme set of molecules stopping 



VIB 



and another 



YOR 



The reason that we get yellow light by reflection is that 

 more of the central light is reflected than is transmitted. 



If we do not consider reflection, the thing becomes 

 simpler: for instance, if I take a tube one foot long and fill it 

 with chlorine gas, and observe the spectrum of a white 

 light through the tube, we find that chlorine absorbs only 



in the blue ; the yellow and red are freely transmitted ; a 

 glass coloured red is so coloured because its molecules 

 absorb the blue, and a blue glass is blue because it absorbs 

 the red. 



Prof. Stokes, in one of his lectures in South Kensing- 

 ton, dealt with the colours of natural bodies in connec- 

 tion with the absorption of light by them, and I may 

 be permitted to close the present paper with the following 

 extract from so great an authority : — 



" What is the cause why a green leaf is green, or why 

 a red poppy is red ? It is frequently said that the reason 

 why a red poppy is red and that a white lily is white is, 

 that the lily reflects rays of all kinds, but the poppy 

 reflects only the red ones, and if you place the red poppy 

 in a pure spectrum it is luminous, like a white lily, in the 

 red ; but if you place it in the green it will be almost 

 black, whereas the white lily Avill be brilliantly green. 

 Now the common explanation, properly understood, is 

 true ; but it is not the Avhole truth, and if understood as 

 it is liable to be understood, it is false. It is true that a 

 red poppy reflects red rays, and a white lily reflects rays 

 of all colours ; but it is not true that the preference for 

 the red to the green in the one case and the equality of 

 action in the other takes place in the act of reflection. It 

 is not a phenomenon of coloration by reflection. The 

 coloured light is reflected or you would not see it ; it is 

 sent out of its course before it enters your eye, and it is 

 true that the light, in its life's history, undergoes reflection ; 

 it is not true that it is in the act of reflection that the 

 one colour gets the preference over the other. Here I 

 have some solution of the colouring matter of green 

 leaves in alcohol, and here is some more alcohol, with 

 which I will dilute the former. I have obtained a beau- 

 tiful green solution, although the green colour is not seen 

 now by reflected, but by transmitted light. As regards 

 the light which falls upon the surface there is a little white 

 light reflected, just a s there would be from water, but 

 very little is reflected from the surface where the fluid is 

 in contact with the glass, the chief portion of that re- 

 flected being from the outer surface of the glass itself. 

 You would not see any green at all in it unless there Avere 

 something placed behind so as to reflect the light back- 

 wards. You see there that the colour of the green leaf, 

 as ordinarily seen, is due to the combination of reflection 

 with the phenomena of absorption, or the swallowing up 

 of certain kinds of light when light is sent through a 

 perfectly clear medium. I may illustrate this in another 

 manner. Here is a vessel of water into which I will pour 

 some blue solution. If I send light through it, it will 

 appear of a deep blue, but if I hinder the light from coming 

 behind, which I can do by putting black cloth behind it, 

 it is simply dark ; you do not see the blue colour at all. 

 Why? Because there is nothing behind to reflect the 

 light. Suppose I make it a little muddy by pouring into it 

 some pounded chalk, you see the blue colour immediately. 

 Why is that ? You know that if powdered chalk were put 

 into water it would not colour the fluid. But here each 

 Uttle particle of uncoloured chalk reflects a small quantity 

 of light falling upon it, so that it fulfils the same office as 

 a mirror placed behind the fluid. You may imagine that 

 the particles of chalk are so many minute mirrors capable 

 of reflecting light. If you take any one particle of chalk, 

 say one-tenth of an inch deep, in the liquid, the light from 

 the sky falls upon the fluid, it undergoes absorption in 

 passing through that first tenth of an inch, and then the 

 portion of light which is left is reflected by that little 

 particle of chalk, and passes out again, and so, as regards 

 that single particle, the light which reaches your eye from 

 beneath that depth has itself gone through a stratum of 

 fluid of one-fifth of an inch in thickness, and accordingly 

 you see the colours produced by selective absorption ; that 

 is to say, by the absorption of certain kinds of light, which 

 are more greedily devoured by the fluid than the other 

 kinds. This is what takes place in the green leaf and in 



