200 
Inquiry into the Causes of Light and Colour . 
of colours ; and the use of mordants in occasioning durability may partly be by 
affording a mechanical cement and partly by chemical compensation, i. e. by supply- 
ing oxygen in proportion as it is abstracted. The difference which sometimes exists 
between reflected aud transmitted light, would depend on the portion of oxygen 
appropriated by the light : in either case colour or no colour would depend on the 
relative affinity of the substance and light for oxygen ; and durability and fugitive- 
ness of colour would turn on the nature of the coloured substance, whether it co- 
operated with light in effecting decomposition or repaired the loss by restoring 
oxygen. 
Newton concludes that the size of the particles which regulated the nature of the 
reflected colour, depended on the density of the reflecting substances ; be therefore 
thought, from their effect on colouring matter, that alkalis were incrassaling, and 
acids attenuating ; and Mr. Delaval, with the view of illustrating and establishing 
the theory of Newton, made an immense number of experiments, for which he was 
honoured by the Royal Society with the Copley gold medal. Doctor Bancroft and 
M. Berthollet controverted the hypothesis of Mr. Delaval ; but although I do not 
believe that colour depends on the density of the reflecting substances, yet it is re- 
markable that Newton’s opinion regarding the properties of alkalis and acids on 
colouring matter is in perfect harmony with those which I ascribed to them, and 
which I attribute to a general law of action in oxygen by which, when it enters into 
such a state of combination as qualifies it for uniting with light and forming colour, 
its effect whether on light or any other substauce is incrassating. 
In investigating the properties of colouring matter I found that, in proportion as it 
combined with oxygen, its colour was deepened and its solubility diminished, which 
would seem to accord with Newton's doctrine concerning the size of the particles j 
since it is to be expected that the size of the particles would correspond to the rarity 
or density of the substance. To take indigo for an instance : fully oxydisedit is dark 
blue, heavy, insoluble; less oxydised lighter blue, specific gravity diminished, and still 
insoluble ; further reduce its oxygen, it is green and soluble, and so on till the colour 
is destroyed. 
My hypothesis merely transfers these phenomena to light : it assumes white light 
to be a chemical compound of phlogiston and the supposed principle ; and that by 
passing through a person or particular media, or by reflection, or transmission by 
particular bodies, it parts with phlogiston and unites with oxygen in certain pro- 
portions ; that these proportions determine the size of the particles, as shown in the 
instance of indigo, and the size of the particles determines the colour. 
Agreeably to this theory the mixture of all colours would not destroy colour, but 
occasion exactly the effect which experience proves that it does. Black would 
not depend on small particles but on the contrary, taking charcoal as evidence ; this 
is the most insoluble substance in nature, it perhaps of all colouring matter contains 
the largest portion of oxygen, and therefore possesses the largest particles, I con- 
ceive the difficulty and inconveniences connected with the black dye or the art of 
dyeing black proceed from these causes, 
^ ^ink that bodies possessing no oxygen (even supposing their existence 
capable of proof j, and yet affording colour (take chlorine as an example which de- 
rives its name from its green colour), would be any valid objection to the hypothesis, 
suppose the whole weight of facts to be in favour of oxygen being the colouring 
principle, and suppose the prism to afford a proof of oxygen passing through trans- 
parent bodies, it would be easy to conceive that bodies possessing no oxygen them- 
selves may yet occasion the chemical union of light and oxygen. 
The property of the prism at its two extremities, even beyond the visible rays, of 
oxygenating and disoxygenating, seems to me to confirm the view I have taken of 
ine power of different media to occasion the union of light with oxygen; and I think 
e f C ^ D i neC ^i° n anc * h ea *» their distinct nature, and all the phenomena pre- 
sented by them are perfectly reconcilable with tiie assumed theory. It is not, 
However by any means intended to contend for the soundness of all the opini- 
ons incidentally introduced. Chemistry i3 scarcely out of her leading strings : 
we see through a glass, darkly ; and it is only by accurate observation, precise 
experiments, and new discoveries that the sciences can hope to acquire strength and 
E.M. 
