340 'CLARKE : CHEMICAL STABILITY 



Molecular saturation, molecular symmetry and simplicity of 

 constitution are favorable to stability; unsaturated compounds 

 or highly complex substances are less likely to be permanent. 

 Stability is also a function of temperture, for one compound 

 may decompose below 100°, while another can be heated to 

 2000° without breaking down. At the absolute zero all com- 

 pounds should be equally stable. Furthermore, stability is con- 

 ditioned by association or environment. Two substances, each 

 stable by itself, may form, when brought together, an unstable 

 system. That is, they react chemically, to generate a new system 

 of greater stability. Here again, temperature is a controlling 

 factor, and pressure also may exert an influence. At very low 

 temperatures, chemical activity ceases, and at very high tem- 

 peratures all compounds are dissociated. Each reaction is possi- 

 ble only within its own range of temperature, which may be 

 low or high. At ordinary temperatures sodium sulfate and 

 silica can remain in contact indefinitely; at a full red-heat they 

 react, the sulfate is decomposed and a silicate is formed. Stabil- 

 ity, then, is a purely relative term, and must always be considered 

 with reference to specific conditions. A substance which is 

 stable under one set of conditions becomes unstable under another. 

 The foregoing observations are obvious and commonplace, 

 but they serve as a preface to something which is less evident 

 and which seems to have received little attention so far. When 

 two elements unite in equal or nearly equal masses, the compound 

 produced is more likely to be stable than when the masses widely 

 diverge. For example, sulfur dioxide, in which the masses of 

 sulfur and oxygen are practically the same, is more stable than 

 the trioxide. When the latter compound is decomposed by heat 

 the dioxide is one of its products, and when sulfur burns in air, 

 the dioxide alone is formed. In carbon monoxide the elements 

 are combined in the ratio 12 to 16; and this substance, despite 

 the fact that it is unsaturated, is more stable than the dioxide 

 with the ratio 12 to 32. Among the oxides of nitrogen the same 

 relation holds, and the compound NO, with the simplest ratio 

 between the masses, is the most stable of all. Other good ex- 

 amples are furnished by cyanogen, 12 to 14, boron nitride, 11 



