CHEMICAL PHILOSOPHY (CONTINUED). 47 



erted by some substances which assist chemical action without themselves 

 undergoing any chemical change. Dissociation is another term applied to 

 a special kind of chemical change effected by heat alone. But a full dis- 

 cussion of these points is here out of place. 



44. Atomicity. After you have become familiar with the multitude 

 of compounds formed by the union of the elementary bodies, it will appear 

 that there is a large class of elements which invariably combine with each 

 other in the proportion of one atom to one atom. Hydrogen, chlorine, 

 bromine, iodine, sodium, potassium, and silver belong to this class ; there 

 are such bodies, for example, as HC1, Agl, KBr, NaCl, etc., in which the 

 elements are combined in the simple ratio of one to one. Moreover, chem- 

 ists are not able to make any such bodies as H 2 C1, or HC1 2 , or NaCl 2 , or 

 K a Br 3 ; hence it is supposed that this class of bodies are monatomic, and 

 are said to possess only one bond of affinity. 



There is a second class of elements which are prone to unite with two of 

 these monatomic elements, and are hence called diatomic, and are said to 

 have two bonds or tint to of affinity. Oxygen, sulphur, calcium, etc., belong 

 to this class ; thus water contains two atoms of H to one of O, and is writ- 

 ten, as you know, H 2 O. We have other examples in the following bodies, 

 CaCl 2 , H 2 S, K 2 O, Na 2 O. These diatomic elements may also unite with 

 two dissimilar monatomic elements, giving rise to such bodies as KHO, 

 NaHS, etc. ; in this case, however, hydrogen is generally one of the mon- 

 atomic elements. 



Besides these monatomic and diatomic elements, there are several other 

 classes the tri-, tetr-, pent-, and hex-atomic which combine respectively 

 with three, four, five, and six monatomic elements. A triatomic element 

 may unite with one monatomic element and one diatomic ; a tetratomic 

 element may combine with two diatomic elements, or with one triatomic 

 and one monatomic element, etc. This combining capacity, or atom-Jixing 

 power, is generally believed to point to a real difference of chemical power ; 

 it has nothing to do with the atomic weights, nor with the combination by 

 volume. 



This idea of atomicity is represented in symbols by a very simple 

 method ; a single stroke attached to the symbol, thus H' or H-, signifies 

 that the element named has only one bond of affinity, or is monatomic. 

 T\vo strokes connected with a symbol, thus O'', or -O-, or O=, represent 

 a diatomic element; three, N'" or X N', a triatomic ; four, C iv or -C-, a tetr- 

 atomic, etc. You may, if you please, regard these strokes as so many arms 

 stretched out to grasp some other element. Water is often represented 



