6 BIOLOGY. [BooK r. 



cannot fix each more than one atom of chlorine or of bromine ; 

 they are monoatomic, as, for instance, hydrogen. Calcium, barium, 

 strontium, in order that their attractive power may be saturated, 

 need to fix two atoms of chlorine ; they are diatomic, as, for 

 instance, oxygen. Phosphorus, which in the perchlorure of 

 phosphorus succeeds in fixing five atoms of chlorine, is pentatomic. 



It is these inequalities in the mode and the power of combina- 

 tion, in the capacity of saturation, which we call the atomicity of 

 each atomical species, designating specially by that expression 

 the maximum capacity of saturation. However, hereby is by no 

 means implied that a pentatomic species, for instance azote, can- 

 not combine with less than five atoms. Azote, which fixes five 

 atoms in the chlorohydrate of ammonia (AzH 4 Cl), is not more 

 than triatomic in ammoniac gas (AzH 3 ), and is only diatomic in 

 the bioxide of azote. For the sake of greater clearness, the 

 denomination atomicity is reserved to designate the capacity of 

 absolute saturation. The capacities of inferior saturations are 

 called quantivalences. Thus then azote is pentatomic, but it is 

 trivalent in gas ammoniac, and so on. 



This notion of atomicity has thrown a great light on the 

 ultimate texture of bodies, and also on the march hither and 

 thither of atoms in various combinations. In effect, free or 

 combined, every atom tends to saturate itself by the annexion of 

 other atoms. If, for instance, a tetratomic atom has combined 

 with two atoms only, it ceases not to tend to saturate its attrac- 

 tive force ; it strives to fix two atoms more. But these two 

 atoms once found, no other simple body can combine with our 

 tetratomic atom, unless by displacing one or two of its atoms 

 and becoming their substitute. If, for instance, we take from 

 a carburet of saturated hydrogen an atom of hydrogen, the 

 molecule thus mutilated can unite itself to an atom of chlorine. 

 But the chlorine is monoatomic ; this, however, does not hinder 

 it from fixing the complex molecule of the carburet, impoverished 

 to the extent of an atom of hydrogen. The reason is that 

 certain atomic groups, certain molecules, can play in combina- 

 tions the part of a single atom. They are what we call com- 



