372 On the component Parts of Light, 
mersed in cold water*, The colour of charcoal, probably, de- 
pends chiefly, cr only, upon its mechanical form. The experi- 
fients on the great power of acquiring heat possessed by the co- 
four black, would also induée the belief, that it usually arises 
from the absorption of light. 
Bodies of every colour, probably, absorb a part of the ethereal 
fatter attracted by them; and as the reflection of the attracted 
hight is from a mechanical cause, the proportion of the two ethe- 
real Substances absorbed, will be the same, or very nearly so, as 
those attracted. This will produce an expansion of the bodies ; 
Wliich ‘expansion cannot with reason be assigned to one only of 
the ‘ethereal fluids, if both be absorbed, but must necessarily arise 
from ‘the whole of the ethereal matter which enters into combi- 
nation. The ethereal fluid therefore, which causes expansion, 
must be of different kinds in different bodies. This may be shown 
by eXperiments. 
_ Ff bodies are luminous from ‘the attraction of the eye for light, 
the colour of the light must depend upon the nature of that at- 
traction ; consequently, all bodies which are luminous only from 
their containme an excess of ethereal matter, as where they are 
so'from temperature, appear'of the same‘colour}. This is liable 
to’some evident modifications. For if by any means, such for 
example as chemical combinations, a quantity of light is disen- 
gaged, the attraction of the eye must be affected by the greater 
facility of obtaining peculiar proportions. And though light is 
never so absolutely disengaged, as to be wholly wnattracted; yet 
substances may, by stronger chemical affinities, have the attrac- 
tion for a portion of the ethereal fluid so weakened, that it may 
be ‘considered, when under the influence of ancther attraction, as . 
if absolutely set free. Different substances, it is well known, im 
their combinations with oxygen, emit light of different colours ; 
and as the ethereal matter in the oxygen must in all cases be of 
the same ndture,'the other substances must have emitted different 
kinds. Those'substances, therefore, must have contained different 
kinds; for they could not have emitted that which they did ‘not 
contain. And as all the ethereal matter which they contained 
must have tended to their degree of expansion, we are brought 
to the same conclusion as before, that caloric, or the ethereal 
eause of expansion, is not of the same nature in all bodies. 
f * Journal of Science and the Arts, No. III. and see the observations 
there. 
’ 4 The difference of the colour of heated solids as their temperature is 
raised, probably arises from a cause before alluded to, the different energy 
of the two rays ; whence it happens, that at first as the light is barely able 
to overcome the difficulties in its reaching the optic nerve, the calorific ray 
is more prevalent, giving the red tinge, which gradually fades to white, as 
the eye is able to acquire the proportions naturally attracted by it. Li i 
° ; sees ight 
