On the Atomic Theory. 87 



whole of the hght which they gave out, so as to prevent illumi- 

 nation ? 



V/e must draw the same line of distinction between free light 

 and combined light that we do between free caloric and com- 

 bined caloric. The light of the sun comes to us partly combined 

 with caloric, but much of it is free. The same may 6e said with 

 respect to the light given out by burning bodies. Caloric de- 

 prives the light, which forms atmospheres round its ultimate di- 

 visions, of the power of illuminating, on the same principle that 

 the ultimate divisions of ponderable matter deprive their natural 

 atmospheres of caloric of the power of heating. 



As all bodies contain caloric, we have every reason to suppose 

 that they also contain the matter of light ; for it is constantly 

 liberated, more or less intensely, during the rapid union of oxy- 

 gen to most inflammable bodies, or by the union of certain in- 

 flammable bodies to each other, such as that of sulphur and cop- 

 per, sulphur and potassium, regains of arsenic and potassium, 

 and potassium and tellurium, &c. without the agency of oxy- 

 gen. 



The brilliancy of the light, however, under all circumstances, 

 depends on the intensity of the heat, or, in other words, npoii 

 the degree of the concent.'-ation of the particles of caloric at the 

 moment of their liberation by the chemical union of ponderable 

 bodies. In proportion as the caloric passes from the focus of its 

 concentration by its particles flying from each other more or less 

 distant, their capacity for absorbing light is proportionally in- 

 creased ; and consequently, as already mentioned, they recover 

 those atmospheres of light which they lost by their concentra- 

 tion. 



The following well-known simple facts tend to support this 

 doctrine ; viz. If a patent lamp, or any other brilliant burning 

 body, be suffered to go on in a small and well-closed room, the 

 temperature of the surrounding space is gradually exalted, at the 

 same time that the light in the room is not in the smallest sen- 

 sible degree increased, because it is absorbed by the expanding 

 caloric as fast as it is liberated from the concentrated caloric, on 

 the same principle that caloric is liberated by the compression 

 of gases, and absorbed again upon restoring them to their former 

 degree of expansion. Thus it is that the light of bodies in the 

 act of brilliant combustion ceases the moment they are extin- 

 guished, without any marked interval ; otherwise there would 

 be a gradual diminution of light in the surrounding space before 

 it totally vanished. 



When a bright sun-beam is suffered to pass into a dark room 



through a small openinir, the instant that opening is closed by 



an opaque substance the light vanishes, because its source is cut 



' F 4 off. 



