98 THE ANALYSIS OF SOILS. V. 



washings are evaporated to complete diyness in a platinum or 

 porcelain basin on the water bath, the residue heated over a 

 flame until thoroughly dry and the organic matter charred or 

 burnt, cooled, moistened with strong hydrochloric acid, taken 

 up with water, and filtered ; the residue is ignited and 

 weighed, its weight, " soluble silica," being added to the other 

 " silica" already separated. 



The nitrate and washings are made up to 100 c.c. (or 250 if 

 more convenient). 25 c.c. (or 50 if from 250 c.c.) are then 

 taken, boiled with a few drops of nitric acid, in order to oxidise 

 the iron, and mixed with just sufficient ammonia to neutralise 

 the free acids present, A slight excess of ammonia is then 

 added and the whole boiled until the free ammonia is nearly 

 all expelled. The precipitate is filtered off, washed, thoroughly 

 dried, ignited, and weighed. The weight is equal to the ferric 

 oxide, alumina, and phosphoric acid present. 



Another portion of the filtrate from the silica is taken for 

 the determination of the total potash. This can be done con- 

 veniently and accurately by Tatlock's method. 



25 c.c. (or 50 c.c.) of the hydrochloric acid solution are 

 placed in a porcelain dish, and platinum tetrachloride in suffi- 

 cient quantity to convert all the potash, soda, and magnesia into 

 double chlorides is added (in general, about '3 gramme will be 

 sufficient), and the liquid slowly evaporated on the water bath. 

 The residue is washed, first with a little platinum chloride 

 solution, which dissolves and removes sulphates, phosphates, 

 &c., also the double chlorides of platinum with calcium, 

 sodium, magnesium, &c., and then by decantation, in the dish, 

 with alcohol (96%, not methylated), the washings being passed 

 through a small filter. When the washings are colourless, the 

 precipitate, w T hich should consist of bright orange crystals, is 

 washed with alcohol into a weighed porcelain crucible, and 

 the alcohol poured off as completely as possible through the 

 filter. The crucible is then placed in a warm place for a short 

 time, heated in a steam bath for two hours, and then weighed. 

 The small filter, W 7 hich should only contain traces of precipi- 

 tate, is then burnt in platinum wire, its ash added to the 

 crucible, and the whole again weighed. The last increase is 

 taken as being due to 2KCl + Pt. It is calculated to K 2 PtCl G 



