35 



into the plant, and if the immediate function of one of the con- 

 stituents of this salt (its chlorine) in the plant is not understood, 

 we see at least a reason for its presence in vegetable food, in 

 the fact that muriatic acid, of which chlorine is an element, is 

 indispensable to the process of digestion in the animal. 



So, if iron be present in vegetable food, it is, among other 

 reasons, because iron is a constituent of the healthy blood; 

 and if manganese be detected in the plant in still smaller quan- 

 tity, it is because this substance also, though in smaller propor- 

 tions than iron, is constantly to be detected in the blood. 



If the roots of plants stretch themselves widely through the 

 soil, it is that they may pick up those substances which, like 

 phosphoric acid, are present in the soil in minute quantity only, 

 and yet are absolutely necessary to vegetable growth. And if 

 this phosphoric acid accumulate, especially in the grain, it is 

 because animals, which chiefly live upon grain, must obtain it in 

 that food in sufficient abundance to supply with readiness the 

 wants of the most rapidly increasing bones. 



And if, again, all the chemical elements of a really nutritive 

 food at one time accumulate in the seeds of plants, at another 

 in their roots or leaves, and at another diffuse themselves 

 throughout the whole substance of the plant, it is that each ani- 

 mal may be supplied with nourishment in that most appropriate 

 form which is adapted to the special structure and digestive 

 powers of its stomach and alimentary canal. 



A thousand such close and striking relations, among all the 

 departments of practical husbandry, will from time to time 

 suggest themselves to the mind of the instructed man ; and 

 while they make his proceedings, especially those of an ex- 

 perimental kind, more interesting to him, they will also clear 

 up many obscurities, and remove many difficulties out of his 

 way. 



The final impression I am desirous of leaving upon the mind 

 of the reader, by all I have said in this preliminary chapter, is 

 this, that experiments by which truth is to be established, or 

 from which natural laws or principles are to be deduced, cannot 

 be hastily or ignorantly undertaken, or thoughtlessly, or with- 

 out much care and anxiety, carried on. This remark, of course, 



