MANUAL OF AGEICULTURE. 225 



and the result is a compound body or substance. The least 

 number of atoms in such an union, or rather, the least quantity 

 of such a compound body, that can be formed, or exist in a free state, 

 is called a molecule; wherefore, chemical compounds are com- 

 posed of an aggregation of molecules. Still more complicated 

 chemical compounds are formed by the union of the molecules 

 of different compound bodies. The propiortion of molecules re- 

 quisite with a given quantity of different molecules, to form a 

 new compound, is called the equivalent of the latter. In all 

 chemical action, heat is given off or taken up; in the process of 

 combination, it is evolved, in that of separation, its absorption 

 is requisite thereto. A certain amount of heat is absorbed by all 

 substances Av^hile they pass from the solid to the liquid, and from 

 the liquid to the gaseous forms, which heat remains in abeyance 

 — latent, until such substances are again transformed into their 

 original forms, and then it is evolved. When the majority of 

 chemical bodie3 assume the solid, instead of the liquid or gaseous 

 form, they appear as small particles of a definite geometrical 

 shape, called crystals, each compound invariably preserving its 

 own peculiarly distinctive crystaline form. Moreover, whatever 

 be the size of any united accumulation of specific crystals, the 

 aggregate mass shapes itself into the geometrical type of its 

 minutest constituent crystal. Chemical bodies not observinji^this 

 law, in the process of their solidification, assume a structureless 

 texture, and are called amoiylious, literally, without form. 

 Again, numerous compounds arising through the agency of animal 

 and plant life, show neither a crystaline nor an amorphous, but 

 a cellular or orf^anised texture. A chemical combination of ele- 

 ments is something quite distinct from mixture pure and simple 

 — that is to say, mere mechanical union. In the latter case, there 

 is no interchange of the several atoms, — no chemical action takes 

 p)lace. There is merely mechanical juxtaposition of particles. 

 By w^ay of illustration, take the preparation of common mortar. 

 There, before the addition of water to the lime shells, the latter 

 are in the state called caustic lime, or oxide of calcium — a com- 

 bination of the simple metallic element calcium, and the simple 

 gaseous element oxygen — one atom of the one in chemical com- 

 bination with one atom of the other ; whence its technical symbol 

 in chemistry Ca. 0. Upon the addition of water, a violent dis- 

 turbance in the mass occurs, together with the evolution of much 

 heat. And this action continues until tlie certain amount of 

 water rer[uisite to enter into combination with all the caustic lime 

 present has been added. Any amount of water superadded 

 thereto, and the sand, are simply mixed with it mechanically. 

 No further immediate chemical action ensues. 



Another illustration of chemical combination is the common 

 class-room experiment, showing the composition of water. 





