286 Mr Ritchie’s Experiments on Radiant Heat. 
atoms of the glass, or at a disadvantageous place, will either be 
reflected or retained by the glass. The reason why caloric, mo- 
ving with a small velocity, passes through very thin glass, whilst 
it is completely stopped by a thicker plate, is hence perfectly 
obvious, the resistance in the one case being much greater than 
in the other. 
This explanation receives additional support from the follow- 
ing curious fact, discovered by De Laroche, that, if invisible 
caloric pass through several very thin plates of glass, a greater 
proportion of it is detained by the first plate than by the second, 
a greater proportion by the second than by the third, &c. 
According to the theory which we have adopted, it is obvious 
that some of the molecules of caloric will be thrown off in more 
advantageous circumstances than others. Some of them will be 
thrown off by an oblique impulse, whilst others will be darted 
forward by those acting directly behind them. Besides, some 
will have their velocities diminished by others crossing their 
paths, while others will have their velocities increased by favour- 
able collisions. Hence it is probable, that the atoms of caloric, 
emanating from the same source, do not all move with the same 
velocity. If this be the case, then it is obvious that all the 
atoms moving with a comparatively small velocity, will be de- 
tained by the first plate, and only those allowed to pass which 
move with a velocity sufficient to overcome the resistance. Now, 
all those which escape the grasp of the first plate, will have a 
velocity sufficient to permeate the second, and consequently a 
much smaller proportion will be detained by it. 
8. If there be any truth in the above theory, then it is ob- 
vious that radiant heat moves with every possible velocity, till it 
verge into that of light. Having arrived at such a velocity, it 
will then pass through the various humours of the eye, and pro- 
duce an impression upon the retina. Hence it is probable that 
light is merely caloric moving with an increased velocity. 
( To be continued.) 
