158 
lours of natural bodies than the several sizes or den- 
sities of their transparent particles *. 
419. These conclusions he drew from observing 
the colours afforded by plates of air and water, sub- 
mitted to compression between two object glasses, 
whereby various colours were made to emerge, in 
successive orders,, according to the degree of pres- 
sure employed. Extending the same principles to 
explain the permanent colours of natural bodies, he 
ascribed the yellow, orange, and red colours which 
certain liquors exhibit at different depths, when view- 
ed in a conical glass by transmitted light, to the vary- 
ing thickness or density of such fluids, which succes- 
sively intercept and extinguish the more refrangible 
rays, until, near the top, they transmit the least re- 
frangible, or red rays alone t In like manner, he 
supposed vegetable infusions to be turned red by 
acids, because it is the nature of acids to dissolve or 
attenuate j and that the same infusions were rendered 
green by alkalis, because these substances precipitate 
or incrassate J. Mr Delaval follows Newton in at- 
tributing these phenomena to the same causes ; and 
more lately, M. Haliy observes, that, in all these 
changes of colour the union of the moleculae of the 
two liquids forms mixed moleculse, the thickness of 
which is different from that of the component mole- 
culse, and determines the reflexion of colour answer- 
ing to that thickness . 
* Optic?, B. ii. part 3. prop. 10. 
f Ibid. B. i. part 2. prop. 10. 
J Ibid. B. ii. part 3. prop. 7. 
Elem. Nat. Phil. vol. ii. p. 243. 
