ORDER OF AFFINITY. 187 



Baiytes is capable of taking sulphuric acid from strontian, 

 potash and every other base which follows it in the table,, the 

 experiment being made upon sulphates of these bases dissolved 

 in water; while sulphate of barytes is not decomposed by any 

 other base. Lime separates ammonia and magnesia from sulphuric 

 acid, but has no effect upon the sulphates of soda, potash, stron- 

 tian and barytes ; and in the same manner any other base de- 

 composes the sulphates of the bases below it in the column, 

 but has no effect upon those above it. Tables of this kind 

 when accurately constructed may convey much valuable in- 

 formation of a practical kind, but it is never to be forgotten 

 that they are strictly tables of the order of decomposition and 

 of the comparative force or order of affinity in one set of 

 conditions only. This will appear by examining how far decom- 

 position is affected by accessory circumstances in a few cases. 



Circumstances ivhich affect the order of decomposition. Vola- 

 tility in a body promotes its separation from others which are 

 more fixed, and consequently facilitates the decomposition of 

 compounds into which the volatile body enters. Hence, by 

 the agency of heat, water is separated from hydrated salts ; 

 ammonia, from its combinations with a fixed acid, such as 

 the phosphoric ; and a volatile acid frcm many of its salts, 

 as sulphuric acid from the sulphate of iron, carbonic acid from 

 the carbonate of lime, &c. Ammonia decomposes hydrochlo- 

 rate of morphia at a low temperature, but on the other hand, 

 morphia decomposes the hydrochlorate of ammonia at the boil- 

 ing point of water, and liberates ammonia, owing to the vola- 

 tility of that body. The fixed acids, such as the silicic and 

 phosphoric disengage in the same way at a high temperature 

 those acids which are generally reputed most powerful, 

 and by which silicates and phosphates are decomposed with 

 facility at a low temperature. Many such cases might be 

 adduced in which the order of decomposition is reversed 

 by a change of temperature. The volatility of one of its 

 constituents must, therefore, be considered an element of in- 

 stability in a compound. 



Decomposition from unequal volatility is, of course, checked 

 by pressure, and promoted by its removal and by every thing 

 which favours the escape of vapour, such as the presence of an 

 atmosphere of a different sort into which the volatile constituent 

 may evaporate. Carbonate of lime is decomposed easily at a 



