CLARKE: CHEMICAL STABILITY 341 



to 14, and sulfur monochloride, 32 to 35.5. Sulfur tetrachloride, 

 32 to 142, is very unstable. In short, stable equilibrium seems 

 to be most easily established when the combining masses approach 

 equality; even though the rule may be modified or apparently 

 abrogated in some cases by other conditions. Such conditions 

 are sufficiently indicated in my prefatory remarks. 



The influence of what, for want of a better name, may be 

 termed mass equilibrium, can be illustrated in many ways. 

 Consider, for example, the several series of normal hydrides; 

 in each series the lowest member is the most stable. Hydrogen 

 combines with fluorine in the cold, and forms a stable compound. 

 "V^ith chlorine the product is also stable, but formed less ener- 

 getically. Hydrobromic acid is not so stable, and hydriodic 

 acid, which can only be prepared by indirect methods, decomposes 

 with great ease. The same rule holds for the series from NH 3 

 to SbHs, and the series H»0 to H 2 Te, with a stable member at 

 one end, and an unstable one at the other. Even the hydrides 

 of the alkaline metals show a similar relation; that of lithium, 

 with the smallest ratio between its components being the most 

 stable. In methane, with the ratio 4 to 12 or 1 to 3 we have a 

 very stable compound, perhaps the most stable or least reactive, 

 chemically, of all the normal hydrides. Among the alkaline 

 oxides that of lithium, ratio 14 to 16, is the one most easily pre- 

 pared and preserved. Examples like these might be multiplied 

 almost indefinitely; provided that the comparisons are made 

 between similar compounds in strictly definite series. When 

 different series are compared other influences may come into 

 play and modify the mass relations. 



A study of solubilities also reveals some regularities which 

 seem to indicate mass relations like those already considered. 

 In many series of compounds the solubility is least when the 

 combined masses most nearly approach equality. The alkaline 

 chlorides and iodides, for instance, show this regularity clearly, 

 as follows: The solubilities represent grams of salt in 100 grams 

 of water at 0°. 



