8 CHEMICAL MANURES. 



The structure of a plant depends on imponderable agents light, 

 heat, electricity. Now, to use these as auxiliaries it is absolutely 

 necessary we know the effects of each. This can only be known by 

 basing our deductions and laws upon the theories which precede 

 them. 



The first question is : Of what is the substance of plants formed ? 

 From whence comes it ? How do the combinations of elements 

 which chemists show, operate ? 



Upon this point Chemistry is as clear as decided. 



She answers : Of fourteen invariable elements, which, for conveni- 

 ence, are arranged in two parallel series : 



Organic Elements. Mineral Elements. 



Carbon, Phosphorus, 



Hydrogen, Sulphur, 



Oxygen, Chlorine, 



Azote. Silicium, 



Iron, 



Manganese, 

 Calcium, 

 Magnesia, 

 Sodium, 

 Potassium. 



Why are the first elements called organic and the second mineral? 

 Because the first are found combined only in living beings, and the 

 second belong by their nature to the solid crust of the earth. 



But how is it, we ask, that so limited a number of elements suffices 

 for so many dissimilar productions ? 



The answer is very simple : Because they possess the power of in- 

 definite combination, like the letters of the alphabet though small in 

 number, yet enough to form all the words of a language. 



Another question arises : Is the composition of a plant the same in 

 all its parts? Do its varied organs differ but in form? Are the 

 stem, the bark, the leaves and the fruit but different impressions of 

 the same substance ? 



Far from that. In a certain degree, each organ has its own com- 

 position. But these variations, the result of conditions absolutely 

 necessary to the reproduction of the species, can be reduced to a few 

 simple propositions. 



We begin with the mineral elements. In general the leafy parts 

 of a plant contain more minerals than do the tougher parts. This is 

 only because the aqueous parts of the sap evaporate quickest in the 

 first organs. 



Evaporation is active in proportion to looseness of tissue and 

 directness of contact with the atmosphere. Thus we find more min- 

 erals in grasses than in trees, more in leaves than in bark, and more 

 in bark than sap-wood or in heart-wood. 



In the fruit of a leguminose there are two distinct parts the shell 

 and the pea. The shell, which is in more immediate contact with 

 the atmosphere than the pea, contains most minerals. Following 



