CH. Ill] WATER-CULTURE. 55 



(75) Potassium salts necessary. 



Take three plants A, B, C, as nearly as possible of 

 equal weight and equally developed. Dry A at 100° and 

 determine its dry weight. Grow B in normal culture 

 solution and C in a fluid containing the same salts as the 

 normal solution but with an equivalent weight of sodium 

 — instead of the potassium — salt. Continue the cultures 

 for about three weeks, then take out the plants B and G, 

 dry them at 100° and determine their dry weights. B 

 should be considerably heavier than G. 



To confirm the fact that the greater increase in 

 weight shown by B is associated with the actual ab- 

 sorption of the potassium, B and G should be incinerated 

 after weighing and the absolute amounts of KaO in the 

 whole ash of each determined. 



Instructions for obtaining the ash and making an 

 accurate estimation of the KjO are given in part II. 



(76) Phosphoric acid necessary. 



The same method is used as in the last experiment 

 but a somewhat longer time will be required for satis- 

 factory results. The solution which contains no salt of 

 phosphoric acid may have the usual calcium phosphate 

 replaced by an equivalent quantity of calcium nitrate. 



Instructions for determining P2O5 in the ash are given 

 in part II. 



In this as the preceding experiment it need scarcely 

 be pointed out that it is much better to start five or six 

 separate cultures under each set of conditions than to 

 rely on one only. If all develope well, the mean result 

 of the best three may be taken in each case. 



