DECOMPOSITION OF WATER. 49 



driven off by the heat and lime remains. (This lime is the 

 oxide of the metal calcium.) When lime is brought into 

 contact with moisture, about one-third of its weight of water 

 is absorbed and the lime, swells, breaks apart and falls into 

 a very fine powder which is perfectly dry. This water is 

 combined with the lime and cannot be expelled at less than 

 a red heat. Gypsum contains in the same manner 21 per 

 cent, of water; alum contains 24 parts of water to two of 

 solid matter ; Epsom salts contain 50.2 per cent, of water ; 

 and so on through a long list of crystallized minerals. It 

 has also a strong affinity for clay and all the more so, as the 

 clay is finely pulverized and disintegrated ; carbonized veg- 

 etable matter also takes up a large quantity of water ; hence 

 the great advantage of securing as large a quantity of de- 

 cayed organic matter, as may be possible in the soil. 



This is quite distinct from the mechanical grasp upon 

 water exerted by porous substances, w r hich merely hold it 

 in its interstices by capillary attraction, as is the case with 

 a sponge, and give it out again with great facility and with- 

 out any chemical action. 



The elements of water are held together loosely and are 

 combined with great ease. When hydrogen is burned in 

 the air it combines w r ith oxygen (as has been previously 

 described) and forms water. If a piece of zinc is placed in 

 a vessel of water a glass bowl or a wide mouthed bottle, 

 for instance and a little sulphuric acid (a few drops) is 

 added, the water is in part decomposed and the hydrogen 

 is set free. As this experiment is a pleasing one and very 

 simple, the chemical operation is here explained. The sul- 

 phuric acid acts upon the zinc and combines with it ; but 

 as this acid has only three equivalents of oxygen (S. Og) 

 and zinc requires one more equivalent to make the combin- 

 ation as sulphate of zinc (Z. 8.04) this excess of oxygen is 

 taken from the water, leaving the hydrogen free, when it 

 escapes in bubbles apparently from the surface of the zinc. 

 If the bottle is corked and a glass or rubber tube is put 

 through the cork, the hydrogen gas may be collected. But 

 as it is explosive when mixed with air, great care must be 



