S84 Mr Ritchie's Experiments and Observations 
by some eminent philosophers in this country, and by others 
denied altogether. It is true, as has been proved by De Laroche, 
that when the glass is thick, and the temperature of the source 
of heat low, caloric will not pass through glass like light ; but 
b^ conducted through it in the same manner as through a plate 
of metal. 
4. Having procured plates of glass of different degrees of 
thickness, I employed them as screens, as in the preceding ex- 
periments, and found, that those plates which stopped the whole 
current of heat when the temperature of the screen was low, al- 
lowed a portion of it to pass when the temperature of the body 
was sufficiently raised, though still invisible in the dark. 
5. We are now prepared to examine into the nature of that 
power by which heat is propelled from the surface of a heated 
body. It is generally admitted, that the atoms of caloric are 
attracted by every other substance, while they mutually repel 
each other. If hot water be poured into a cold vessel, the 
sides of the vessel, having a greater attraction for caloric than 
the water has, will gradually conduct a portion of it to the ex- 
terior surface. When the first range of calorific atoms has 
reached the exterior surface of the vessel, those directly behind 
exerting their repulsive energy, will propel them with a velocity 
proportional to this power. Hence, if the temperature of the 
body be raised, the exterior film of caloric will consist of a pro- 
portionally greater number of calorific atoms, and consequently 
the quantity of radiant heat will also be proportionally increased. 
If the mutual repulsive energy of the calorific atoms, and the 
attraction between these atoms and those of the heated body, re- 
mained constant at every temperature, then would the quantity 
of radiant heat increase in geometrical progression, agreeably to 
the law suggested by Sir Isaac Newton. But this is by no means 
the case. For, by condensing caloric in a given body, the mu- 
tual repulsive energy of the atoms is increased, while the attrac- 
tion of the body for caloric is diminished ; — hence the flow of 
heat from the surface of a body wall increase in a much higher 
ratio than the increase of temperature. This fact, deduced from 
the theory which we have shortly stated, was observed long ago 
by Dr Martine and Erxleben, and has been ably investigated 
by Dulong and Petit. 
