118 THE ART OF CULTURE. 



we must admit that the amount of phosphates in the above men- 

 tioned mineral waters is very considerable. It may be shown 

 by calculation that the water of the fountain at Carlsbad 

 must take up many thousand pounds of phosphate of lime in its 

 passage through the layers of rocks. 



A few very simple experiments point out the manner in which 

 the earthy phosphates, and particularly phosphate of lime, are 

 taken up by the roots of plants. 



Phosphate of lime is insoluble in pure water, but it dissolves 

 readily in water containing common salt, or a salt of ammonia ; 

 and in water containing sulphate of ammonia it dissolves as 

 readily as gypsum. Phosphate of lime is also soluble in water 

 containing carbonic acid ; in this respect it is analogous to car- 

 bonate of lime. 



The soil in which plants grow furnishes their seeds, roots, and 

 leaves, with phosphoric acid, and they in turn yield it 'to 

 animals, to be used in the formation of their bones, and of those 

 constituents of the brain which contain phosphorus. We may 

 form an idea of the quantity of phosphate of magnesia contained 

 in grain, when we consider that the concretions in the caecum 

 of horses consist of phosphate of magnesia and ammonia, which 

 must have been obtained from the hay and oats consumed as 

 food. Twenty-nine of these stones were taken after death from 

 the rectum of a horse belonging to a miller, in Eberstadt, the 

 total weight of which amounted to 3 lbs. ; and Dr. F. Simon has 

 lately described a similar concretion found in the horse of a 

 carrier, which weighed 1£ lbs. 



Some plants extract other matters from the soil besides 

 silica, the alkalies, alkaline earths, sulphuric and phosphoric 

 acids, which are essential constituents of the plants ordinarily 

 cultivated. These other matters, we must suppose, supply, in 

 part at least, the place, and perform the functions, of the sub- 

 stances just named. We may thus regard common salt, nitre, 

 chloride of potassium, and other matters, as necessary constitu- 

 ents of several plants. 



Clay-slate contains generally small quantities of oxide of 

 copper ; and soils formed from micaceous schist contain some 

 metallic fluorides. Now, small quantities of these substances 



