10 CHYMISTRY APPLIED TO AGRICULTURE. 



and which are rendered solid by being brought into com- 

 bination with other substances, return to their natural 

 state as soon as a sufficient degree of heat is applied to 

 destroy the force of the affinity which unites them to their 

 base. But those which are not originally gaseous in their 

 form, under the influence of heat pass through all the 

 degrees intermediate between their natural state and that 

 of imperceptible vapor ; and return to the concrete state 

 when deprived of the excess of heat which had been ap- 

 plied to them. 



Caloric can be extracted from bodies by percussion or 

 compression, in the same manner as water is expressed 

 from a substance which has imbibed it. When a body is 

 deprived of its caloric by either of these means, the mole- 

 cules composing it are brought closer to each other, and 

 its porosity, and consequently its volume, diminished. 

 The act of striking or rubbing hard bodies together, 

 produces the same effect ; the portion of caloric, which is 

 in either case set free, acts as heat. 



The temperature of bodies can be lowered, or elevated, 

 by placing them in contact with other bodies more or less 

 hot than themselves. The flnlrl of heat will pass from 

 one to the other, and produce an equilibrium in their 

 state, according to their respective capacities for caloric, 

 which enable them to absorb unequal quantities of it. 



All bodies in their natural state contain a determinate 

 portion of caloric ; but when their density undergoes a 

 change, by the variations of temperature to which they 

 are exposed, they lose or absorb caloric, in proportion to 

 their contraction or dilation. The gases, which become 

 solid by entering into combinations, the vapors which are 

 condensed, the solids which are contracted, impart to the 

 air a portion of their caloric, which becomes heat ; whilst 

 all these bodies, on receiving heat from the air, are 

 dilated. 



The phenomena of composition and decomposition, 

 which uninterruptedly renew the surface of our globe, 

 give rise every moment to the emission or absorption of 

 caloric. Two substances, entering into combination, form 

 a compound which perhaps requires more or less caloric 

 than is contained in the two component principles; and 

 then either heat or cold is necessarily produced during 

 the operation. Those gases, which become solid by en- 

 tering into combination, part with their caloric whilst un- 



