2 HYDKOGEN 



Such digressions are inevitable, if clear and adequate explanations of 

 many natural processes are to be attempted. 



In the application of chemistry to agriculture cases often arise in 

 which the truth of the old adage, " A little knowledge is a dangerous 

 thing," becomes strikingly apparent, and the conclusions arrived at from 

 the consideration of a particular problem from the standpoint of ordin- 

 ary elementary chemistry, are quite opposite to the results of actual 

 practice. Such contradictions arise, not from any inaccuracies in 

 general principles, but through leaving out of consideration the effects 

 produced by some apparently insignificant circumstances or conditions. 



It is evident, therefore, that although there is no distinct agricultural 

 chemistry, yet the problems which arise in agriculture demand a know- 

 ledge of chemistry in which due attention is given to the peculiar cir- 

 cumstances under which the reactions take place. 



In this .work it will be assumed that the reader possesses an ac- 

 quaintance with general elementary chemistry and is familiar with the 

 properties of the more commonly occurring elements and their chief 

 compounds. 



The student of agricultural chemistry soon finds that of the eighty 

 odd elements which are known, only a comparatively small number, 

 some twelve or fourteen, are concerned in most of the changes which 

 are brought before his notice. It may perhaps be advisable to very 

 briefly remind the reader of the properties of these important elements, 

 the mode of their occurrence, and the characteristics of some of their 

 compounds. The elements which are most important to living or- 

 ganisms are hydrogen, oxygen, nitrogen, carbon, sulphur, phosphorus, 

 potassium, sodium, calcium, magnesium, iron, silicon, aluminium, 

 chlorine and fluorine. 



Hydrogen. This substance, as its name implies, is a constituent 

 of water. Its most important chemical properties are its strong ten- 

 dency to combine with oxygen, the act of union being accompanied 

 by the evolution of a large amount of heat, and its power of uniting in 

 a vast number of different proportions with carbon, to form that very 

 numerous and important group of bodies known as the hydrocarbons. 

 It also enters into the composition of almost all compounds existing 

 in the bodies of plants and animals, i.e., into nearly all forms of or- 

 ganic matter. Its atomic weight is the smallest of all the elements and 

 was formerly taken as unity, but now it is more usual to take oxygen 

 = 16 as the basis of atomic weights, that of hydrogen on this scale 

 being about I'OOS. Consequently, though the proportion by weight of 

 hydrogen in the substances comprising the crust of the earth is small, 

 yet the number of atoms of hydrogen actually existent and taking part 

 in the changes going on, must be very large compared with those of 

 other elements apparently (and by weight) much more abundant. 

 Take water for example here the hydrogen by weight constitutes 

 only -i of the total and the oxygen ^> yet there really are twice as 

 many atoms of hydrogen as of oxygen, as indicated by the formula 

 H 2 O. In reality, the relative amounts of elements present in any 

 system, so far as their chemical activity is concerned, ought to be 



