56 BORDER LINES OF KNOWLEDGE 



element wMcli is not a natural constituent of the 

 vegetable structiu'e, except perhaps externally, for the 

 accidental purpose of killing parasites. The whole 

 art of cultivation consists in learning the proper food 

 and conditions of plants, and supplying them. We 

 give them water, earths, salts of various kinds such 

 as they are made of, with a chance to help themselves 

 to air and light. The farmer would be laughed at 

 who undertook to manure his fields or his trees with 

 a salt of lead or of arsenic. These elements are not 

 constituents of healthy plants. The gardener uses the 

 waste of the arsenic furnaces to kill the weeds in his 

 walks. 



If the law of the animal cell, and of the animal 

 organism, which is built up of such cells, is like that 

 of the vegetable, we might expect that we should treat 

 all morbid conditions of any of the vital unities be- 

 longing to an animal in the same way, by increasing, 

 diminishing, or changing its natural food or stimuli. 



*' That is an aliment which nourishes ; whatever we 

 find in the organism, as a constant and integral ele- 

 ment, either forming part of its structure, or one of 

 the conditions of vital processes, that and that only 

 deserves the name of aliment." * I see no reason, 

 therefore, why iron, phosphate of lime, sulphur, should 

 not be considered food for man, as much as guano or 

 poudrette for vegetables. Whether one or another of 

 them is best in any given case, — whether they shall 



* Lewes, Physiology of Common Life, I. 76. 



