and the Cause of Colour. 371 
nation of the proportions reflected forms their colours. This is 
strongly confirmed by the fact, that where no chemical change 
takes place, the mixture of different coloured substances, such as 
igments, invariably produces some intermediate colour; that 
is, that two substances attracting different proportions, when 
mixed, always attract proportions compounded of those two. 
The absolute visibility of objects, therefore, may be supposed 
to arise from their attracting the calorific and chemical matter 
from adistance, which are banned as they are reflected. Their 
colour depends upon the preportions attr acted, and its bright- 
ness upen the quantity reflecied, which is praduved by the foree 
of the attraction, and the polish of the body. The degree of vi- 
sibility of bodies depends upon the force of the attraction, the 
polish of the body, and the equality of the proportions of the 
ethereal fluids, — 
The colour black may arise from the attraction of either one 
of the ethereal substances, or from the absorption of all which it 
may have attracted. It is probable, however, that neither of 
these cases is ever complete} and that the old opinion, that 
there is no perfect blackness, is correct. Darkness is the same 
sensation in the eye as blackness; and so evidently so, that al- 
most all the words in the language applicable to the one, are 
used to describe the other. Perfect blackness, therefore, would 
probably render an cbject as invisible as total darkness. The 
contrast. however, between light aid shadow, and whiteness and 
blackness, frequently makes the apparent effect complete; and in 
each case, that depth of shade and colonr appears perfect, which 
a more careful examination shows to be widely removed from it: 
while no known palpable substance is absolutely invisible, except 
from its transparency; consequently, none can be perfectly black. 
The more frequent cause of the common degree of that colour, 
seems to be the absorption of nearly all the light attracted ; and 
this is sometimes occasioned by mechanical construction. It 
has. been observed, by Thenard, that when a piece of phosphorus 
of a pale yellow colour is nielted in hot water, and then plunged 
into cold water, it becomes perfectly black; but it recovers its 
original colour, on being re-melted. Dr. Brewster also, in pre- 
paring prisms of realgar, by melting it between two plates of 
glass, found that this mine ral, which has naturally a dark orange 
colour, became of a darker eolbar as the heat increased, till ak a 
certain temperature it became perfectly black. — It alwavs, how- 
ever, resumed its original colour upon cooling, even though im- 
calorific particles are the larger; and if the force of every equal part be 
equal, the united power of the parts of the larger particle will be move 
effectual than the divided action of an equal bulk “composed of smaller par- 
ticles. 
Aa2 mérsed 
