292 Dr Davy''s Agricultural Discourse. 



and delicate layer, imparting great power of resistance, and 

 far less soluble when acted on by rain, than the less common, 

 or at least less abundant, phosphate of lime. Animals, on 

 the contrary, being able to range abroad in quest of food, 

 select such as contain phosphate of lime and azote, and in 

 such compounds as admit of digestion and assimilation, and 

 conversion into bone and muscle, following, in so doing, their 

 natural tastes, undoubtedly instinctively directed. 



Leaving these general views, it may be well to consider 

 the subject we have entered upon somewhat in its details. 



Physiologists who have directed their attention especially 

 to the food of animals, have arrived at the conclusion, that, 

 amidst the extraordinary variety of articles capable of sup- 

 porting animal life, there are three w^hich may be considered 

 as of most importance, and, as it were, elemen^tary alimen- 

 tary substances — substances which are found in milk, viz., 

 an albuminous matter, the curd ; an oily matter, the cream ; 

 a saccharine matter, the sugar-of-milk. It seems to be 

 proved by a wide induction of facts, that articles containing 

 these substances, or their analogues, such as starch for sugar, 

 muscle for curd, any kind of fat for cream, are fit for the food 

 of animals generally, and that no articles are fit that do not 

 contain more or less of these. These substances, taken into 

 the stomach, are converted into a pultaceous semifluid 

 chyme, from whence a milk-like chyle is formed, and from 

 whence blood, by which every part of the body is nourished, 

 in its constant circulation. 



The results of the inquiries of physiologists, as regards 

 the food of vegetables, have not been so well defined and sa- 

 tisfactory. As the sap of plants is a fluid, and transparent, 

 we are sure that complete solution is essential as a prelimi- 

 nary, and that nothing enters the spongioles of the roots, or- 

 ganic in its structure — a state of perfect solution being incom- 

 patible with such structure. The principal part of the sap is 

 water ; in it are dissolved carbonic acid, phosphate of lime, 

 carbonate of lime, carbonate of potash, and in very many in- 

 stances silica. And these inorganic substances, I appre- 

 hend, are to the plant for its food what the organic sub- 

 stances before mentioned are to the animal for the same 

 purpose ; and these are not less elementary than those, as 



