40 Report op the Acting Director and Chemist of the 



in small quantities, it does not follow that their presence is of less 

 importance ; in their absence, vegetation would disappear. 



We will now consider each of these elements in order, and men- 

 tion briefly some of the more important characteristics of each; but 

 before doing this, it is desirable to explain the meaning of two or 

 three chemical terms which we shall have occasion to use. 



Classification into Acid-forming Elements and Metals. — 

 Of the fouiteen elements which are found in plants, some are 

 spoken of as non-metallic elements or acid-forming elements, because, 

 in certain combinations, these elements form well-known acids. The 

 other elements are known as metallic elements or inetals. 



{a) Acid-forming Elements. 

 Carbon. 

 Hydrogen. 

 Oxygen. 

 Nitrogen. 

 Phosphorus. 

 Sulphur. 

 Chlorine. 

 Silicon. 



(J) Metals. 

 Calcium. 

 Potassium. 

 Sodium. 

 Iron. 



Magnesium. 

 Manganese. 



Acids and Salts. — {a) An acid is a compound containing am, 

 acid-forming element combined with hydrogen and oxygen., or in 

 some cases, with hydrogen alone. The following examples will 

 serve to illustrate : 



Nitrogen with oxygen and hydrogen ioYva% nitric fl^c^6^ (aquafortis). 



Phosphorus with oxygen and hydrogen iorm^ phosphoric acid. 



Sidphiir with oxygen and hydrogen forms sidphuric acid (oil of 

 vitrol). 



Chlorine and hydrogen form hydrochloric acid (muriatic acid). 



(^) A Salt is a compomul formed by p>\Ltting a metal in the place 

 of the hydrogen of an acid ; that is, an acid differs from a salt 

 simply in having a metal where the acid has hydrogen. Every acid 

 has a salt corresponding to it. For example, as stated above, nitric 

 acid consists of nitrogen and oxygen and hydrogen. Now, if we 

 put the metal potassium in the place of hydrogen we have a com- 

 pound containing 



Nitrogen and oxygen and potassium (in place of hydrogen). This 

 compound is the potassium salt of nitric acid and is called potas- 

 sium nitrate or nitrate of potash. Again, 



