THE FOOD OF PLANTS. 77 



40 mm. long and broad and 5 mm. thick. They are supported by 

 threads fastened to sticks which rest on the rims of the beakers. 

 If the preparations are allowed to stand for some time, it is found 

 that the solution in a does not become turbid ; neither does that 

 in 6, since the marble becomes covered with a crust of Calcium 

 oxalate, which arrests the further action of the Oxalic acid. The 

 fluid in c also remains clear, but in d and e considerable preci- 

 pitates of Calcium oxalate are thrown down. In d the Nitric acid 

 acts on the marble. It gives rise to Calcium nitrate, which now 

 is decomposed by the Oxalic acid present, with formation of 

 Calcium oxalate and liberation of Nitric acid. This latter then 

 acts again on the marble. If, as is actually the case, a con- 

 siderable precipitate is produced in e, it can only be explained as 

 due to the liberation of Nitric acid by the action of the Oxalic 

 acid on the Potassium nitrate present in the solution, this Nitric 

 acid then behaving like that in d. These reactions in d and e pro- 

 ceed so rapidly, as I have convinced myself, that even in the 

 course of half an hour a considerable amount of Calcium oxalate 

 separates out. We can therefore in the above manner demon- 

 strate quite conveniently even in lecture the fact that Oxalic acid 

 is able to decompose Potassium nitrate. 



1 For further information respecting nitrate decompositions consult Emmer- 

 ling, Vcrsuchsstationen, Bd. 17 and 30. 



III. THE CONSTITUENTS OF THE ASH OF PLANTS 



26. Mechanical Analysis of Soil. 



In judging of the character of a soil, it is of great importance 

 to ascertain exactly the amount of coarser and finer elements in 

 it. We therefore distinguish, as has long been customary, be- 

 tween the skeleton and the fine earth of the soil. The latter 

 demands some special attention, since it determines the most 

 important characteristics of the soil, and chiefly furnishes the 

 plant with the mineral matter which it requires for its nutrition. 

 To obtain fine earth for the purpose of chemical analysis, air-dry 

 soil is riddled through a sieve with meshes 0'5 mm. wide. But 

 if it is desired to determine the quantity of fine earth contained in 

 a soil, we treat 50 gr. of air-dry earth in a dish with water, heat 

 on the water-bath for about an hour, and then transfer to a sieve 

 with 0'5 mm. meshes, the fine earth being washed through, 

 first with the help of a paint brush and finally by means of a fine 



^^^f> 



UNIVERSITY 



