ON WATER AND ITS O >.M P< )TNDS 85 



reason a gas may be entirely expelled from a gaseous solution by 

 boiling at least, in many cases when it does not form particularly stable 

 compounds with water. In fact the surface of the boiling liquid will 

 be occupied by aqueous vapour, and therefore all the pressure acting 

 on the gas will belong to the aqueous vapour. Consequently, the partial 

 pressure of the dissolved gas will be very inconsiderable. For this, and 

 for no other reason, a yas separates from a solution on boiling the liquid 

 holding it. At the boiling point of water the solubility of gases in 

 water is still sufficiently great for a considerable quantity of a gas to 

 remain in solution. The gas dissolved in the liquid is carried away, 

 together with the aqueous vapour ; if boiling be continued for a long 

 time, then in the end all the gas will be separated. 37 



which the carbonic anhydride is then dissolved will be (supposing that the common 



JQ ~ 



pressure remains constant the whole time) equal to OQ_~> consequently there is not in 

 solution 18 c.c of carbonic anhydride (as would be the case were the partial pressure 

 equal to the atmospheric pressure), but only 18 2Q _ , which is equal to x, and conse- 



I Q ~ 



quently we obtain the equation 18 ou_ =#> hence # = 8'69. Again, where the atmo- 

 sphere into which the gaseous solution is introduced is not only that of another gas but also 

 unlimited, then the gas dissolved will, on passing over from the solution, diffuse itself 

 through this atmosphere, and from its limitedness produce an infinitely small pressure 

 in the unlimited atmosphere. Consequently, no gas can be retained in solution under 

 this infinitely small pressure, and it will be entirely expelled from the solution. For 

 this reason water saturated with a gas which is not contained in air, will be entirely de- 

 prived of the dissolved gas if left exposed to air. Water also passes off from a solution 

 into the atmosphere, and it is evident that there might be such a case as a constant 

 proportion between the quantity of water vaporised and the quantity of a gas expelled 

 from a solution, so that not the gas alone, but the entire gaseous solution, would pass off. 

 A similar case is exhibited in solutions which are not decomposed by heat (such as those 

 of hydrogen chloride and iodide), as will afterwards be considered. 



37 However, in those cases when the variation of the co-efficient of solubility with the 

 temperature is not sufficiently great, and when a known quantity of aqueous vapour 

 and of the gas passes off from a solution at the boiling point, an atmosphere may be 

 obtained having the same composition as the liquid itself. In this case the amount of 

 gas passing over into such an atmosphere will not be greater than that held by the 

 liquid, and therefore such a gaseous solution will distil over without change, and without 

 altering its composition during the whole period of boiling or distillation. The solution 

 will then represent, like a solution of hydriodic acid in water, a liquid which is not 

 changed by distillation, while the pressure under which this distillation takes place re- 

 mains constant. Thus in all its aspects solution presents gradations from the most feeble 

 affinities to examples of intimate chemical combination. The amount of heat evolved in 

 the solution of equal volumes of different gases is in distinct relation with these variations 

 of stability and solubility of different gases. 22 '3 litres of the following gases (at 700 

 mm. pressure) evolve the following number of (gram) units of heat in dissolving in a 

 large mass of water ; carbonic anhydride 5,600, sulphurous anhydride 7,700, ammonia 

 8,800, hydrochloric acid 17,400, and hydriodic acid 19,400. The two last-named gases, 

 which are not expelled from their solution by boiling, evolve approximately twice as 

 much heat as such gases as ammonia, which are separated from their solutions by boiling, 

 whilst gases which are only slightly soluble evolve less heat than the latter gases. 



