ON WATER AND ITS COMPOUNDS 69 



substance of the earth's surface, and of plants and animals. There 

 is so much water on the earth's surface, that were the surface of sub- 

 stances formed of soluble matters it would constantly change, and 

 however substantial their forms might be, mountains, river banks and 

 s-u shores, plants and animals, or the habitations and coverings of men, 

 could not exist for any length of time. 22 



of 7<>0 mm. by one volume of a liquid. If n cubic centimetres of water absorb m cubic 



centimetres of a gas, then one cubic centimetre absorbs . If c.c. of a gas are ab- 



n n 



sorbecl under a pressure of It mm., then, according to the law of the variation of 

 solubility of a ^as with the pressure, there would be dissolved, under a pressure of 



760 mm., a quantity varying in the same ratio to as 760 : h. In determining the 



residual volume of gas its moisture (note 1) must be taken into consideration. 



Below are given the number of grams of several substances saturating 100 grams of 

 water that is, their co-efficients of solubility by weight at three different temperatures : 



At At 20 At 100 



, Oxygen, O 2 

 Carbonic anhydride, CO 2 



"^fe ^ _ 



I Ammonia, NH 3 I 90-0 51-8 7'3 



, Phenol, C fi HO ! 4'9 5'2 oo 



Liquids Ainyl ak-oliol, C-.H^O . . . . 4'4 2'9 



^ Sulphuric acid, H,>SO 4 .... oo OO OO 



( Gypsum, CaSO 4 , 2HoO . j ^ i ^ 



] Alum, AlKSoOg , 12H..O ...... | 8'3 15'4 857'5 



Solids - Anhydrous sodium sulphate, NaoSO 4 



Common Salt, NaCl 

 Nitre, KN0 3 



4-5 20 



85-7 86-0 



18-8 81-7 



43 



39'7 



Sometimes a substance is so slightly soluble that it may be considered as insoluble. 

 Many such substances are met with both in solids and liquids, and such a gas as oxygen, 

 although it does dissolve, does so in so small a proportion by weight that it might be 

 considered as zero did not the solubility of even so little oxygen play an important part 

 in nature (as in the respiration of fishes) and were not an infinitesimal quantity of a gas 

 by weight so easily measured by volume. The sign QO, which stands on a line with sul- 

 phuric acid in the above table, indicates that it intermixes with water in all proportions. 

 There are many such cases among liquids, and everybody knows, for instance, that spirit 

 {absolute alcohol) can be mixed in any proportion with water. Common corn spirit 

 (vodky) is a mixture of about fifty parts by weight of pure spirit to 100 parts by weight 

 of water. 



22 Just as the existence must be admitted of substances which are completely un- 

 decomposable (chemically) at the ordinary temperature for there are substances which 

 are entirely non-volatile at such a temperature (as wood and gold), although capable of 

 decomposing (wood) or volatilising (gold) at a higher temperature so also the existence 

 must be admitted of substances which are totally insoluble in water without some degree 

 of change in their state. Although mercury is partially volatile at the ordinary tem- 

 perature, there is no reason to think that it and other metals are soluble in water, alcohol, 

 or other similar liquids. However, mercury forms solutions, as it dissolves other metals. 

 On the other hand, there are many substances found in nature which are so very 

 slightly soluble in water, that in ordinary practice they may be considered as insoluble 

 <for example, barium sulphate). For the comprehension of that general plan according to 

 which a change of state of substances (combined or dissolved, solid, liquid, or gaseous) 



