vi] METHODS OF ANALYSIS 167 



filter paper, which has been dried at ioo to a constant 

 weight. Ten c.c. of 95 per cent, alcohol containing 

 02 per cent, of perchloric acid is now added, and 

 the insoluble potassium perchlorate transferred by 

 means of it to the weighed filter paper. In washing 

 the last traces of precipitate on to the filter another 

 20 or 30 c.c. of the alcohol containing perchloric acid 

 are used, and finally the perchloric acid itself is washed 

 out of the filter paper by using a minimum of 95 per 

 cent, alcohol, until the washings do not react with 

 sensitive litmus paper. On weighing, each gram of the 

 perchlorate = 0-3401 K 2 0- 



The other determinations which may be made in 

 this solution consist of soda, lime, magnesia, iron, 

 alumina, manganese, and sulphuric acid, but in most 

 cases these may be omitted. It is occasionally desirable 

 to examine the soluble salts in the soil ; about 200 

 grams of the fine earth should be successively washed 

 with small portions of hot water by the aid of a 

 filter-pump. In the solution the total solids are 

 determined ; they consist, in the main, of the nitrates, 

 sulphates, and chlorides of sodium, potassium, mag- 

 nesium, and calcium, which can be determined by 

 the usual methods. Of course, the amount of soluble 

 salts to be found in the surface soil at any time is 

 largely regulated by the previous weather ; after con- 

 siderable rainfall the soluble salts are washed down 

 into the subsoil, after long evaporation they are con- 

 centrated in the surface layers. The amount of nitrates 

 that is present is further affected by the previous 

 cropping, temperature, and working of the soil, and 

 by the manipulation the soil receives after it reaches 

 the laboratory. Thus the determination of the soil 

 constituents that are soluble in water does not enter 

 into the ordinary routine of analysis, their presence 



