1 CHEMICAL MANURES. 31 



Before finishing this point let us say a word on the nature of these 

 two bodies. 



Clay is a hydrated silicate of aluminum, the proportion of water 

 in it being very variable, running from ten to twenty-five per cent, 

 of its weight. 



Clay has its origin in the silicates of eruptive rocks. You will 

 perhaps find it difficult to believe that granite and porphyry, which 

 are synonymous with resistance and durability, sometimes change 

 with astonishing facility. When the cooling of these rocks is too 

 sudden, they experience a kind of exterior exfoliation from the effects 

 of the weather, consequent upon which their earthy and alkaline 

 bases, potash, soda, lime, etc., are washed off by the rain, while the 

 aluminum remains in combination with a part of the silicate, and 

 forms the clay which we know. 



The nature of sand is more simple : it is essentially formed from 

 silicate in the form of quartz ; it belongs to the great family of 

 arenaceous rocks, which are themselves but blocks of eruptive or 

 volcanic rocks washed off and divided by the action of water. 



Thus, clay owes its origin to the chemical decomposition of these 

 rocks, and sand to the trituration resulting from water, of which the 

 wash of our rivers gives us daily examples. 



The soil contains still another product, humus, very different from 

 the preceding, and to which until recently agriculturists have 

 wrongly given a r61e of the first order. You know that the earth of 

 the heath is essentially formed of sand, and contains a black matter 

 besides. This black matter is insoluble in water, but becomes soluble 

 if a small quantity of caustic potash is added to the water. "Well, 

 this black matter, which we also find in the liquor of manure and in 

 natural earths in very unequal quantities, is humus. 



The following is the composition of humus : C 24 , H 9 , O 9 that is tc 

 say, humus is composed of carbon, hydrogen and oxygen, in the rela- 

 tion to form w r ater, and consequently enters into the frame of the hy- 

 drates of carbon : cellulose, sugar, starch represent, as you know, 95 

 per cent, of the weight of vegetation. Humus has for its origin the 

 same substance with plants, but a species of spontaneous decompo- 

 sition has caused it to lose a certain quantity of hydrogen and oxygen 

 in the form of water. 



The two formulae following are to show the mode of generation of 

 humus : 



Cellulose, C 24 , H 20 , O 20 . 

 Humus, C 24 , Hg, O 9 . 



I tell you, gentlemen, many of the best minds have placed the 

 humus in the first rank as an agent of fertility, but if you ask proofs 

 as a support to this opinion, they can give you none. Vegetable 

 nutrition is an extremely complex phenomenon, of which analysis 

 has shown us very little until the last ten years. When sufficient 

 facts were wanting to define it, they were supplied by hypothesis and 

 words. The word humus has had the happy privilege of eerring as 

 an explanation to all which was not understood. 



