226 MANUAL OF AGRICULTURE. 



wherein the two gases oxygen and hydrogen are mixed in the 

 proper proportions in a vessel. St'ill there is but mechanical 

 union till a flame is applied, when the two gases instantaneously 

 explode with violence, and the chemical combination will be 

 found to have produced an entirely new body, a liquid- water. 



The elements are, according to their possession of certain phy- 

 sical properties, arbitrarily divided into two classes — the metals 

 and the non-metals. And those, again, which are met with in 

 the combination present in animal and vegetable life, are further 

 classified into two groups, viz., organic and inorganic. 



The organic, in all substances, are capable of being separated 

 from the inorganic, and driven off in the shape of gases, by simple 

 combustion. The inorganic always remain, as the ashes of the 

 substance consumed. In the organic elements life, animal or 

 veojetable, mav be said to have had its seat. 



The terms, however, have another application, — " organic" 

 being used in the case of those complicated comjDounds whereof 

 the element carbon is an invariable constituent ; " inorganic," in 

 that of the simpler compounds, with fewer atoms composing their 

 molecules. Chemical compounds, according to their marked 

 characteristics, are all classified under three divisions, viz., acids, 

 bases, and salts. The first two exhibit cj^uite different properties; 

 but when, under certain conditions, they are brought into contact, 

 they lose their distinguishing properties and unite to form the 

 neutral compounds of the third division. 



Particular acids are stronger than others, and can, so to speak, 

 expel the weaker — those acids which have a less degree of affinity 

 for the base — from the salts, and occupy the place in their stead. 

 From a limited point of view, the most characteristic acids may 

 be said to possess a sour taste, and the property of turning a 

 solution of blue litmus to a red colour; whilst, on the other 

 hand, the most marked bases, such as potash, soda, ammonia, and 

 lime — alkalies so called — can restore the solution of blue litmus 

 thus reddened to its original colour, and they have a peculiar 

 soapy taste. To the chemist, however, the terms " acids " and 

 "base" imply the possession of properties of a much wider and 

 less limited description. 



Out of all the chemical elements, about 18 only are discover- 

 able in the blood and tissues of man and the lower animals, and 

 in the juices and fibres of plants. Hence, these should exist in 

 greater or less quantity in our cultivated soils, seeing that animal 

 life depends primarily upon vegetable life, and that the latter 

 again derives its main sustenance directly from the soil. 



Of these 18, none exist in the free state, but as various com- 

 pounds, in bodies animal and vegetable. They are : — 



1st. Organic, comprising Oxygen, Hydrogen, Nitrogen, and 

 Carbon; and 



