CHEMICAL LAWS INVARIABLE. 39 



around the sun; but it also rules the formation of an atom 

 of salt from its component parts ; or the structure of the mi- 

 nute cells of a plant. Everything in nature is subservient 

 to this unchanging law, that certain causes produce certain 

 effects ; which are constant and invariable under all circum- 

 stances. The farmer then as he cultivates his soil and 

 watches the growth of his crops, knows that when he does 

 his part ; which is to prepare the soil properly, and to fur- 

 nish whatever is required to fertilize it and feed the growing 

 plants ; nature provides the unfailing rules by which eifect 

 is given to his work. There is no chance or uncertainty 

 about it ; but the same surety that the results will appear in 

 due time as that the sun will set in the west when the day 

 is done, and will rise in due time to the fraction of a second 

 in the morning. While the combinations of the four or- 

 ganic elements with each other are almost endless, there are 

 but few which contribute directly to the growth of plants. 

 Of these, carbonic acid, nitric acid, ammonia and water, are 

 of the greatest importance; others being of less interest. 

 The atmosphere, however, which is a mixture of the two 

 most important elements, and which is the grand purveyor 

 of vegetable life, contributing from 90 to ' 95 per cent, of 

 their bulk to living plants, requires careful study and 

 notice. 



