620 



THE AQBICULT 



URAL JOURNAL. 



combined state and in its compounds it 

 forms about half the entire weight of the 

 world. The air we breathe consists 

 chiefly of a mixture (not a compound) of 

 oxygen and another gas called nitrogen, 

 the former making up about one-fifth of 

 the total volume ; eight-ninths of water 

 by weight is oxygen ; and it forms a 

 greater or lesser part of the majority of 

 the rocks of which the earth is composed. 

 It is a very active element, combining 

 readily with most other elements, and the 

 compound formed when oxygen unites 

 with another element is called an oxide of 

 that element. Thus the oxygen of the 

 air combines with iron, forming iron 

 oxide, which we know by the name of 

 rust ; oxygen and phosphorus form phos- 

 phoric oxide ; oxygen and calcium, cal- 

 cium oxide or lime ; oxygen and hydro- 

 gen, hydrogen oxide, which is water. It 

 is the oxygen of the air that enables 

 things to burn ; the burning of a candle, 

 for instance, is in fact the combination of 

 the oxygen of the air with the carbon and 

 hydrogen of the grease. The various 

 oxides are of great importance, and some 

 of them will be referred to later on. 



Hydrogen. 



This element is another gas, and the 

 lightest of known substances, being 

 15 times as light as air. The next time 

 we want to make some " killed spirits " 

 for soldering, let us put the pieces of zinc 

 and spirits of salt into a wide mouthed 

 bottle (not too large), and cover the 

 mouth loosely with a piece of paper. 

 After the effervescence has proceeded for 

 a few minutes, apply a light to the mouth, 

 and we shall either get quite a loud pop, 

 or a bluish, almost invisible, flame will 

 burn at the mouth of the bottle. The gas 

 which causes the effervescence is hydro- 

 gen ; if it has completely filled the bottle 

 it will burn quietly, if it is still mixed 

 with air it will explode. In l)oth cases 

 the application of flame has caused the 

 hydrogen to combine with the oxygen of 

 the air and form water, which is, as ex- 

 plained above, the oxide of hydrogen. 



Acids and Bases. 



We shall now be in a position to under- 

 stand the constitution of a large and im 

 portant class of compounds termed acids 

 These are distinguished by a sonr taste 



like vinegar or lemon-juice, by the power 

 of reddening paper dyed blue with litmus 

 (a colouring matter extracted from certain 

 kinds of lichen), andhy always containing 

 hydrogen that cam he replaced by metals. 

 For instance, sulphuric acid consists of 

 hydrogen, sulphur, and oxygen ; if the 

 hydrogen is expelled and its place taken 

 by iron, so that the new compound con- 

 tains iron, sulphur and oxygen, we get 

 what is called a salt of iron and sulphuric 

 acid, to which the name of iron sulphate 

 is given, and all compounds of metals de- 

 rived in this way from sulphuric acid 

 are called sulphates of those metals 

 Similarly, nitric acid contains hydrogen, 

 nitrogen and oxygen : if the hydrogen is 

 replaced by the metal sodium we get 

 sodium nitrate (nitrate of soda), and all 

 such derivatives of nitric acid are called 

 nitrates. In the same way, from phos- 

 phoric acid we obtain salts called phos- 

 phates ; from carbonic acid, carbonates ; 

 from silicic acid, silicates ; and from 

 muriatic acid, muriates : the latter acid is, 

 however, now more properly kncvvn as 

 hydrochloric acid, and its salts as chlo- 

 rides. It differs from the other acids 

 named in containing no oxygen. When 

 we were preparing our soldering fluid just 

 now, what really happened was this : the 

 metal zinc turned out the hydrogen from 

 the spirits of salt (which is the popular 

 name of hydrochloric acid) and took i's 

 place, forming the salt of zinc and hydro- 

 chloric acid, namely, zinc chloride, thus : 



We had at first — 



Hydrochloric acid and Zinc 



composed of 



hydrogen) ^^^^^ ^.^^^^ 

 chlorine ) 



What was lefc was 



Zinc Chloride and Hydrogen 



composed of 



. i and hydrogen, 

 chlorine j 



It is plain that one acid can form a 

 number of different salts, according as the 

 hydrogen is replaced by various metals. 



Most acids are formed by the combina- 

 tion of certain oxides with water ; thus 

 sulphuric acid is the result of an oxide of 

 sulphur combining with Avater ; and such 

 oxides have the name of acid-forming 



