114 MASSACHUSETTS HORTICULTURAL SOCIETY. 



mere names for what we do not comprehend. Phosphoric acid 

 and lime will combine in two proportions only. Two atoms of 

 each form phosphate of lime. Three atoms of phosphoric acid and 

 one of lime form superphosphate. Nobody can tell why they may 

 not as well combine in other proportions. To change phosphate 

 of lime, which is said not to be readily soluble, into superphos- 

 phate, we make use of the chemical affinities of the substances. 

 We have seen that in phosphate of lime, the phosphoric acid and 

 lime are chemically combined in the proportion of two and two. 

 The}' form a sort of equal union or marriage — one bone if not one 

 flesh. Now to break up this union and entice away from the phos- 

 phoric acid the lime, which may be regarded as the weaker vessel, 

 we bring into her neighborhood an object of stronger affection or 

 affinity. Sulphuric acid and lime are found to have a stronger 

 affinity for each other than phosphoric acid and lime, and by add- 

 ing sulphuric acid, we entice away a portion of the lime, forming 

 the new combinations, sulphate of lime and superphosphate. 

 Why. these affinities exist, and why these new combinations are 

 formed, it is as difficult to explain, as why one young gentleman 

 and lady have a stronger affection for each other than for all the 

 world beside. 



What new combinations take place in the soil after our fertilizers 

 are applied, we can only guess, and many substances produce 

 effects entirely beyond our comprehension. Two hundred pounds 

 of gypsum applied to an acre, ten inches in depth of which will 

 weigh two million pounds, have on some soils produced an almost 

 magical effect ; yet I suppose chemistry has thus far failed to solve 

 the mystery. It is thought that the elements that compose gyp- 

 sum do not enter into the composition of the plants, but its effect 

 is due to its chemical action upon other nutritive elements, proba- 

 bly potash and magnesia, rendering them available to the growing 

 crop. Liebig says that he found by experiment that a solution of 

 gypsum in contact with arable soils underwent decomposition, and 

 formed combinations quite contrary to the ordinary affinities as ob- 

 served in the laboratory. 



Among the ideas most common with those who discuss the 

 growth and feeding of plants is this, that the roots can only take 

 up their food in a form in which it is dissolved in water, and the 

 word soluble is usually understood as meaning soluble in water. 



Liebig's opinion seems to be this, and it appears to be entirely 



