84 PRINCIPLES OF CHEMISTRY 



large, yet, as the solubility of oxygen in water is twice that of the 

 nitrogen in water, the proportion of oxygen dissolved in water will be 

 greater than its proportion in air. It is easy to calculate what quantity 

 of each of the gases will be contained in water, and we will take the 

 most simple case, and calculate what quantity of oxygen, nitrogen, and 

 carbonic anhydride will be dissolved from air having the above com- 

 position at and 760 mm. pressure. Under a pressure of 760 mm. 1 

 cubic centimetre of water dissolves 0*0203 cubic centimetre of nitrogen, 

 or under the partial pressure of 600 mm. it will dissolve 0*0203 x Jg#, 

 or 0*0160 cubic centimetre ; of oxygen 0*041 1 x 1 : ", or 0*0086 cubic cen- 



0*4 



timetre ; of carbonic anhydride 1*8 x~ - or 0*00095 cubic centimetre; 



760 



consequently, 100 cubic centimetres of water will contain at altogether 

 2*55 cubic centimetres of atmospheric gases, and 100 volumes of air 

 dissolved in water will contain about 62 p.c. of nitrogen, 34 p.c. of 

 oxygen, and 4 p.c. of carbonic anhydride. The water of rivers, wells, 

 <tc., usually contains more carbonic anhydride. This proceeds from 

 the oxidation of organic substances falling in the water. The amount 

 of oxygen, however, dissolved in water appears to be actually about ^ 

 the dissolved gases, whilst air contains only 1 of it by volume. . 



According to the law of partial pressures, whatever gas be dissolved in 

 water will be expelled from the solution in an atmosphere of another gas. 

 This depends on the fact that gases dissolved in water escape from it 

 in a vacuum, because the pressure is nil. An atmosphere of another 

 gas acts like a vacuum on a gas dissolved in water. Separation then 

 proceeds, because the molecules of the dissolved gas no longer impinge 

 upon the liquid, are not dissolved in it, and those previously held in solu- 

 tion depart from the liquid in virtue of their elasticity. 3 '"' For the same 



3(5 Here there may be, properly speaking, two cases : either the atmosphere surround- 

 ing the solution may be limited, or it may be proportionally so vast as to be unlimited, 

 like the earth's atmosphere. If a gaseous solution be brought into an atmosphere of 

 another gas which is limited for instance, as in a closed vessel then a portion of the 

 gas held in solution will be expelled, and thus pass over into the atmosphere surrounding 

 the solution, and will evince its partial pressure. Let us imagine that water saturated 

 with carbonic anhydride at and under the ordinary pressure be brought into an 

 atmosphere of a gas which is not absorbed by water; for instance, that 10 c.c. 

 of an aqueous solution of carbonic anhydride be introduced into a vessel holding 

 10 c.c of such a gas. The solution will contain 18 c.c of carbonic anhydride. The 

 expulsion of this gas goes on until a state of equilibrium is arrived at. The liquid 

 will then contain a certain amount of carbonic anhydride, which is retained under 

 the partial pressure of that gas which has been expelled. Now, how much gas will 

 remain in the liquid and how much will pass over into the surrounding atmosphere ? 

 In order to solve this problem, let us suppose that x cubic centimetres of carbonic 

 anhydride are retained in the solution. It is evident that the amount of carbonic anhy- 

 dride which passed over into the surrounding atmosphere will be 18 a", and the total 

 volume of gas will be 10 + 18 a: or 28 # cubic centimetres. The partial pressure under 



