ATOM. In chemistry, the ultimate part of a body which combines with other atoms. Theo retically, these are of a determinate magnitude in every case. Atoms are simple or elementary when they can not be separated by cheinical forces, and compound when they are liable to decomposi tion. Chemical compounds consist of a definite number of atoms, bound together by chemical force or affinity; but the value of this force is different in different compounds. In conse quence, however, of the union of atoms in invari able weights, determined by experiment, each chemical body has attached to it a distinct pro portional weight, termed its atomic weight, equiva lent, or combining number. The study of these is the essential of all chemical inquiries; it is this remarkable adherence to a prccise quantity in all cases of combination which gives exactness to our investigations, and forms the difference be tween a mere mixture and chemical union- The following are the atomic weights or proportionals of various elementary bodies interesting to agri culture, and their initial equivalent or contrac tion: Hydrogen ..... 1. Potassium (s.) 89.
Oxygen (0) 8. Sodium (Na.) 23.
Nitrogen (N.) 14. Calcium (Ca. 20.5 Carbon (C) 6. Magnesium (tIg.). 12.7 Sulphur (S.) Aluminium (AI.) 18.7 Phosphorus (P.) 31. Iron (Fe.) 27.
Chlor ne (Cl.) 35.5 Maganese (Mu.).
Silicon (Si.) 22 These are on the basis that hydrogen is 1., and may be understood by the following case. Water is a compound of one atom of hydrogen and one atom of oxygen; and, supposing a given quantity weighs nine grains, we know, by the laws of chemi cal combinations, that it contains one grain of hydrogen and eight grains of oxygen; or, if the weight of water be other than nine grains, these constituents are united in the rigorous proportion of one to eight. Atomic theory is the theory of Dr. Dalton, that chemical union takes place only in definite atoms. Atomic weight is the equiva lent or combining weight.