70 I'KIXCIPLKS OF CHEMISTRY 



Substances which are easily soluble in water bear a certain resrin 

 blance to it. Thus sugar and salt in many of their superficial features 

 remind one of ice. , Metals, which are not soluble in water, have 110 

 points in common with it, whilst on the other hand they dissolve each 

 other in a molten state, forming alloys, just as oily substances dissolve 

 each other ; for example, tallow is soluble in petroleum and in olive oil, 

 although they are all insoluble in water. From this it is evident that 

 the analogy of substances forming a solution plays an important part, 

 and as aqueous and all other solutions are liquids, there is good reason to 

 believe that in the process of solution solid and gaseous substances 

 change in a physical sense, passing into a liquid state. These con- 

 siderations elucidate many points of solution as, for instance, the vari- 

 ation of the co-efficient of solubility with the temperature and the evo- 

 lution or absorption of heat in the formation of solutions. 



The solubility that is, the quantity of a substance necessary for 

 saturation varies with the temperature, and, further, with an increase 

 in temperature the solubility of solid substances generally increases, and 

 that of gases decreases ; this might be expected, as solid substances by 

 heating, and gases by cooling, approach to a liquid or dissolved state. 23 

 A graphic method is often employed to express the variation of solu- 

 bility with temperature. On the axes of abscissae or on a horizontal 

 line, temperatures are marked out and perpendiculars are raised corre- 

 sponding with each temperature, whose length is determined by the 

 solubility of the salt at that temperature expressing, for instance, one 

 part by weight of a salt in 100 parts of water by one unit of length, 

 such as a millimetre. By joining the summits of the perpendiculars, 

 a curve is obtained which expresses the degree of solubility at different 

 temperatures. For solids, the curve is generally an ascending one i.e.* 

 recedes from the horizontal line with the rise in temperature. These 

 curves clearly show by their inclination the degree of rapidity of increase 

 in solubility with the temperature. Having determined several points 



takes place, it is very important to make a distinction at this boundary line (on approach- 

 ing zero of decomposition, volatility, or solubility) between an insignificant amount and 

 zero, but the present methods of research and the data at our disposal at the present 

 time do not yet touch such questions. It must be remarked, besides, that water in a 

 number of cases does not dissolve a substance as such, but acts on it chemically and forms 

 a soluble substance. Thus glass and many rocks, especially if taken as powder, are 

 chemically changed by water, but are not directly soluble in it. 



23 Beilby (1883) experimented on paraffin, and found that one cubic decimetre of solid 

 paraffin at 21 weighed 874 grams, and when liquid, at its melting-point 88, 788 grams, at 

 49, 775 grams, and at 60, 767 grams, from which the weight of a litre of liquefied paraffin 

 would be 795-4 grams at 21 if it could remain liquid at that temperature. By dissolving 

 solid paraffin in lubricating oil at 21 Beilby found that 795'6 grams occupy one cubic 

 decimetre, from which he concluded that the solution contained liquefied paraffin. 



