6 BIOLOGY. [Book i 



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

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

 strontium, in order that their attractive power may be saturated 

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

 instance, oxygen. Phosphorus, which in the perchlorure o 

 phosphorus succeeds in fixing five atoms of chlorine, is pentaiomii 

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

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

 each atomical species, designating specially by that expressioi 

 the maximum capacity of saturation. However, hereby is by m 

 means implied that a pentatomie species, for instance azote, can 

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

 atoms in the chlorohydrate of ammonia (A^H*C1), is not mon 

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

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

 denomination atomicity is reserved to designate the capacity o 

 absolute saturation. The capacities of inferior saturations an 

 called quantivalences. Thus then azote is pentatomie, but it ii 

 trivalent in gas ammoniac, and so on. 



■ This notion of atomicity has thrown a great light on tin 

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

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

 combined, every atom tends to saturate itself by the annexion o: 

 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 twc 

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

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

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

 a carburet of saturated hydrogen an atom of hydrogen, th< 

 molecule thus mutilated can unite itself to an atom of chlorine 

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

 it from fixing the complex molecule of the carburet, Lmpoverished 

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

 certain atomic groups, certain molecules, can play in combina 

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



