48 THE BEGINNINGS Of LIFE. 



addition of calcic chloride, whilst nitre does the same 

 on the addition of alcohol. Greater solubility implies 

 greater chemical affinity, under the influence of which, 

 the molecules of water, leaving those of the substance 

 first dissolved, may combine with the new molecules, 

 whilst the old are free to aggregate in the form of a pre- 

 cipitated salt 1 . The case is only a little more complex 

 where what is called c double decomposition' takes place. 

 But facts of a slightly different nature must also be 

 borne in mind. Some salts which are capable of re- 

 maining in solution together at certain temperatures, 

 may be incapable of doing so when the solution is 

 not maintained at a temperature within this range. In 

 such a case, one of two things may happen : either one 

 of the two salts originally dissolved may be precipitated, 

 or else a c double decomposition' may take place lead- 

 ing to the deposition of one of the alternate salts, 



1 In the highly interesting memoir already referred to, Prof. Tyndall 

 says : ' Carbonic acid is decomposed by the solar beams in the leaves 

 of plants ; but here it is in presence of a substance chlorophyll, ready, 

 as it were, to take advantage of the loosening of the atoms by the 

 solar rays. The present investigation has furnished numerous cases of a 

 similar mode of action. All the vapours examined may be more or less 

 powerfully affected in their actinic relations by the presence of a second 

 body with which they can interact. The presence, for example, of 

 nitric acid, or of hydrochloric acid, may either greatly intensify or greatly 

 diminish the visible action of the light on many vapours decomposable 

 alone or when mixed with air; while the presence of the one or the 

 other of the same acids may provoke energetic action in substances which 

 are wholly inactive when left to themselves.' Nitrite of amyl, nitrite of 

 butyl, and lienol afford good examples of this mode of action, which is 

 very similar to that referred to in the text, and which is often instru- 

 mental in aiding fermentative changes. 



