INTRODUCTION 7 



of the equation ; but it is not like an algebraic equation, in 

 that it cannot necessarily be written either way. An equation, 

 to a chemi-t, is a concise statement of a chemical change, 

 given in addition to information as to its qualitative character, 

 full details of the quantities of the various substances taking 

 part in the reaction. The student, however, must not over- 

 look the fact that before an equation can be relied upon 

 actually to represent a given reaction, a complete and careful 

 experimental investigation of the reaction must have been 

 made. By the beginner particularly, equations must not be 

 used as a means of predicting the interaction of two or more 

 substances, but should be regarded as records of facts which 

 have been ascertained by careful experiment. 



Before giving a brief description of the elements which are 

 of importance in agriculture it may be advisable to explain 

 the meaning of some of the commoner terms used in chemistry 

 As these explanations cannot claim to be other than frag' 

 mentary and disconnected, no attempt at a logical arrange- 

 ment is made. For convenience in reference an alphabetical 

 order has been adopted. 



Acid' — a substance generally possessing a sour taste and 

 the property of changing vegetable blues, e.g.^ blue litmus, to 

 red, and containing one or more atoms of hydrogen, which can 

 be replaced by a metal. As types of acids, sulphuric acid, 

 HjSO^, nitric acid, HNO,, hydrochloric acid, HCl, acetic acid, 

 HCjHjOg, may be mentioned. By replacement of the hydrogen 

 of these substances by a metal, e.g., sodium, the salts sodium 

 sulphate, Na,SO^, sodium nitrate, NaNO,, sodium chloride, 

 NaCl, and sodium acetate, NaCjHgOj, are obtained. The 

 possession of the sour taste and the power of changing vege- 

 table blues to red is indicated by saying that the substance 

 has an acid reaction, but is not a proof that it is an acid ; e.g.y 

 copper sulphate, CuSO^, has an acid reaction, yet it is a true 

 salt. 



Alkali : — '1 . substance opposed in its properties to an acid, 

 capable of neutralising and destroying the characteristics of 



