[ 12 ] 
thicknefles. 3. The fame appears from the table 
(p. 206.) in which the thickneffes of air, water, 
and glafs, and the colours produced by them are 
fet down. 
Thefe experiments are applied by him to tranf- 
parent bodies and the colours exhibited by them; 
but they are equally applicable to permanently 
coloured bodies : and it appears from them, that 
denfer fubftances ought, by their greater rejiedtive 
power, in like circumftances, to refledl the lefs re- 
frangible rays, and that fubftances of lefs denjity 
Ihould reflect rays proportionably more refrangible, 
and thereby appear of feveral colours in the order 
of their denfity. 
In confirmation of this reafoning, I fliall give 
inflances of natural bodies, which differ from each 
other in denfty, though circumftanced alike in 
other refpeds ; and fhall fhew that they differ in 
colour, in the fame order they do in denfity, the 
denfefi being red, the next in denfity orange, 
yellow, &c. 
In fuch an inquiry metallic bodies feem to de- 
ferve our frfl and principal attention, as their 
fpecific gravities have been afeertained by well 
known and repeated experiments. Without enter- 
ing into a minute chemical theory of the princi- 
ples of metals, it is fufficient to obferve that they 
are univerfally allowed to confift of i. an inflam- 
mable or fulphureous matter, which is of the fame 
kind in all the metals ; 2. of a fixed matter or 
calx, which appears in each of the metals to be 
fpecifically different in weight, as well as in other 
properties. 
' 2 Af 
