162 
thinner masses, they do not vary the species of their 
colour, but only transmit a diluter, or more intense 
red *. We may therefore venture to conclude, that 
the opinion of Newton, respecting the colours pro- 
duced in vegetable infusions by acids and alkalis, 
was derived, merely by analogy, from what he 
had observed to happen in liquors apparently of the 
same nature, but which act in a very different man- 
ner on light ; and this difference, we may add, fur- 
nishes a new proof that chemical composition, not 
less than mechanical constitution, influences the per- 
manent colours of natural bodies. 
425. Sir Isaac Newton himself, indeed, may be 
considered as admitting the influence of chemical 
composition in varying the action of bodies upon 
light; for, in stating his general law, he makes a 
well-known exception with respect to inflammable 
bodies, which, according to him, refract light more 
powerfully than other bodies of the same density. 
So much stress, indeed, did he lay on the great re- 
fractive power of these bodies, that he even thought 
it " rational to attribute the refractive power of all 
Bodies, chiefly, if not wholly, to the sulphurous (i. e. 
the inflammable) parts with which they abound ; for, 
adds he, it is probable that all bodies abound more 
or less with sulphurs f." " And this great man," 
says Dr Bancroft, " having also concluded that the 
permanent colours of natural bodies were analogous to 
those produced by the refractions of thin, colourless, 
* MancK Mem. vol. ii. p. 235. 
f Optics, I'i ii. part 3. prop. 10. 
