that for every (dilute) solution the rat io bet wee ti the diminution of vapour 



tension and of the free/in^- point is also a sufficiently constant quant it v.' 1 

 1 he diminution of the vapour tension of solutions explains the rise 

 in boiling point through the solution of soli-,i inui- volatile bodies in 

 water. The temperature of a vapour is the same as that of the solu- 

 tion from which it is generated, and therefore it follows that the 

 aqueous vapour ^i\en oil' from a solution will be superheated. A 

 saturated solution of common salt boils at 1 US 1 .a solution of :'>:$."> 

 parts of nitre in 100 parts of water at 11-V[I . and a solution of '.\'2~> 

 parts of potassium chloride in 1(H) parts of water al 17'. 1 . if the tempera- 

 ture of ebullition be determined bv immersing the thermometer bulb in 

 the liquid itself. This is another proof of the bond which exists between 

 water and the substance dissolved. And this bond is seen still more 

 clearly in those cases (for example, in the solution of nitric or formic 

 acid in \\ater) where the solution boils at a higher temperature than 

 either water or the volatile substance dissolved in it. For this reason 

 the solutions of certain u'a>es for instance, hydriodie or hydrochloric 

 acid boil above 100'. 



The separation of ice from solutions '- explains both the phenome- 

 non, well known to seamen, that the ice formed from salt water gives 

 fresh water, and also the fact that by free/ing, just as by evaporation, 

 a solution is obtained which is richer in salts than before. This is 

 taken advantage of in cold countries for obtaining a liquor from sea- 

 water, which is then evaporated for the extraction of .salt. 



< >n the removal of part of the water from a solution (hv evaporation 

 or the separation of ice), there should be obtained a saturated solution, 

 and then the substance dissolved should separate out. Solutions satu- 

 rated at a certain temperature should also separate out a corresponding 

 part of the substance dissolved if thev be reduced, by cooling, ' 3 to a 



. . at all note :;:'. . and is not proportional to the quantity of the -ul, -lance added. As 



lie. the ten-ion - then equal, accordiut,' to the law of Dalton. to the sum of the 



ten-ion- oi the -uh-tanees taken. Therefore, liquids which are ins,, ] u hle in each other 



i for example, water ,md chloride of earhom pre-eiit a tension eipial to the -urn of their 



dual ten-ions, and the'-efore -udi a mixture hoils at a louei temperature than the 



' : It. in our example, the fall of tension ! di\ ided 1-y the tei - on of water, a figure is 

 ol.taim i .\liielii- nearh llir, t iinc- le than t he magnitude of 1 . i temperat lire of 



M] the application ot the mechanical theory of heat, and is repeated l,\ man\ imesti^'ated 



it t 



