32 Experiments and Observations on the Solar Rays. 
which ascribes opacity to a multitude of reflections within the in= 
ternal parts of bodies, he says: “If the particles of light were 
reflected or merely turned out of their direction by the action of 
the particles, it seems to be quite demonstrable that a portion of 
the most opaque matter, such as charcoal, would when exposed 
to a strong beam of light, become actually phosphorescent during 
its illumination, or would at least appear white.’’* 
The circumstance of the caloric’s existing in a free state in the 
pores of black bodies, and becoming latent in white bodies, af- 
fords a ready explanation of the difference of heat acquired by a 
white and a black body, when both are equally exposed to the 
sun’s rays. It is free, uncombined caloric which occasions the 
sensation of heat, causes the mercury to rise in the tube of the 
thermometer, overcomes the cohesive attraction of the particles 
of coal, &c. and enables them to combine with oxygen and pro- 
duce the phenomenon of ordinary combustion. 
Philosophers, for a long time, supposed the action of natural 
bodies upon light to be purely mechanical. Sir Isaac Newron, 
finding the least parts of many bodies to be in some measure 
transparent, supposed the colors of all bodies to be produced by 
their power of reflecting some rays and transmitting others, and 
their “opacity to arise from a multitude of reflections caused 
within their internal parts.” “'The transparent parts of bodies,” 
says that great philosopher, “according to their several sizes, re- 
flect rays of one color, and transmit those of another, on the same 
ground that thin plates or bubbles do reflect or transmit these 
rays; and this I take to be the ground of all their colors.”’*+ 
Sir Davip Brewsrer, although he doubts not that the “ Newto- 
nian theory is applicable to colors of the wings of insects, the 
feathers of birds, the scales of fishes, the oxidated films on metal 
and glass, and certain opalescences, still believes that the colors 
of vegetable life, and those of various kinds of solids, arise from 
a specific attraction which the particles of these bodies exercise 
over the differently colored rays of light.” “When,” says he, 
“colored bodies are opaque, so as to exhibit their colors princi- 
pally by reflection, the light which is reflected back to the obser- 
ver, has received its color from transmission through part of the 
* Brewster’s Opt. chap. xvi. t Opt. chap, xxxrv. 
