518 AGRICULTURAL, ANALYSIS 



deposition of these salts. In the natural deposition it is noticed 

 that there are often compounds found which do not appear in lab- 

 oratory experiments but the deposition of which can be theoreti- 

 cally ascertained. By change of temperature, especially going 

 above the fixed standard of 25 on which the previous predictions 

 have been based, new combinations of minerals take place and 

 minerals which are formed at 25 disappear. The dominant fac- 

 tors, therefore, in the order of crystallization and the rate of de- 

 position, are temperature and concentration. The variations in 

 pressure to which the salts are subjected doubtless have some 

 influence but these influences are very minute when compared 

 with those above mentioned. 



The agricultural analyst who desires to study the causes which 

 produce these deposits and the order in which the phenomena 

 occur, is directed in the above epitome of van't HofTs work to 

 the principles which guided his investigations. 



Further details of the method of procedure are found in the 

 paper cited. 



439. Potash from Feldspathic Rocks. The question of the 

 availability of potash in feldspathic rocks has been under discus- 

 sion at intervals for many years. The fact that in all virgin 

 soils potash was originally derived from the disintegration of the 

 rocks is incontestable. It has, however, been generally supposed 

 that such long periods of time were necessary in order to carry 

 on the decompositions, that feldspar could not be made practical 

 use of as a potash fertilizer. This impression has been confirmed 

 by the fact that ground feldspar on digestion with water yields 

 only extremely small amounts of potash to the solvent. The sys- 

 tematic investigations of Cushman 10 have developed facts of im- 

 portance in this connection. 



Cushman has shown that a typical pegmatitic feldspar contain- 

 ing 9-3 P er cent, of potash (K 2 O) when ground to a 2OO-mesh 

 powder, yielded the following amounts of potash to solution in 

 water, under varying conditions: 



10 Bureau of Chemistry, Bulletin 92, 1905; Office of Public Roads 

 Circular 38 : 1905. 



