66 PRINCIPLES OF CHEMISTRY 



Just as damp air may be added to any quantity of dry air it be 

 desired, so also a solvent liquid may be taken in an indefinitely large 

 quantity and yet a uniform solution will be obtained. But more than 

 a definite quantity of aqueous vapour cannot be introduced into a 

 certain volume of air at a certain temperature. The excess above the 

 point of saturation will remain in the liquid form.- The relation 

 between water and substances dissolved in it is similar. More than a 

 definite quantity of a substance cannot, at a certain temperature, dis- 

 solve in a given quantity of water ; the excess does not unite with the 

 water. Just as air or a gas becomes saturated with vapour, so water 

 becomes saturated with a substance dissolved in it. If an excess of a 



20 A juxtaposition of (chemically or physically) reacting substances taken in various 

 states for instance, some solid, others liquid or gaseous is termed ft heterogeneous system. 

 Up to now it is only systems of this kind which can be subjected to d- 'tailed examination 

 in the sense of the mechanical theory of heat. Solutions present liquid homogeneous 

 systems, which as yet are subjected to investigation with difficulty. 



In the case of limited solution of liquids in liquids, the difference hi-firci'it tJn 

 and the substance dissolved is clearly seen. The former (that is. the solvent) may be 

 added in an unlimited quantity, and yet the solution obtained will always be uniform, 

 whilst of the substance dissolved there can only be taken a definite saturating propor- 

 tion. We will take water and common (sulphuric) ether. On shaking the ether with the 

 water it will be remarked that a portion of it dissolves in the water, forming a solution. 

 If the ether be taken in such a quantity that it saturates the water and a portion of it 

 remains undissolved, then this remaining portion will act as a solvent, and water will 

 diffuse through it and also form a saturated solution of water in the ether taken. Thus 

 two saturated solutions will be obtained. One solution will contain ether dissolved in 

 water, and the other solution will contain water dissolved in ether. These two solutions 

 will arrange themselves in two layers, according to their density; the ethereal solution 

 of water will be on the top, as the lightest, and the aqueous solution of ether at the 

 bottom, as the heaviest. If the upper ethereal solution be poured off from the aqueous 

 solution, any quantity of ether may be added to it; this shows that the dissolving sub- 

 stance is ether. If water be added to it, it is no longer dissolved in it : this shows that 

 water saturates the ether here water is the substance dissolved. If we act in the same 

 manner with the lower layer, we shall find that water is the solvent and ether the sub- 

 stance dissolved. By taking different amounts of ether and water, the degree of 

 solubility of ether in water, and of water in ether, may be easily determined. Thus, for 

 example, in the above case it is found that water approximately dissolves ^ of its 

 volume of ether, and ether dissolves a very small quantity of water. Let. us imagine that the 

 liquid poured in dissolves a considerable amount of water, and thai water dissolves a 

 considerable amount of the liquid. For instance, let us imagine that the saturation of 

 100 parts of water require 80 parts of the liquid, and that 100 parts of the liquid would 

 require 125 parts of water for its saturation. What would then take place if the liquid 

 be poured iu water ? Two layers could not be formed, because the saturated solutions 

 would resemble each other, and therefore they would intermix in all proportions. 

 Indeed, in the saturated aqueous solution there, would be 0'8 parts of the liquid taken to 

 1 part of water, and in the solution of water in the liquid taken there would be on 

 saturation 1 part of water to 0'8 parts of the liquid. There would be no line of demarca- 

 tion between the layers of the liquids, or, in other words, they would intermix in all 

 proportions. This is, consequently, a case of a phenomena where two liquids present 

 considerable co-efficients of solubility in each other, but where it is impossible to say what 

 these co-efficients are, because it is impossible to obtain a saturated solution. 



