44 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



the molecules, therebj' weakens their cohesive force. Pulverizing, by 

 mechanically separating the molecules to a certain extent, also assists 

 solution. 



Heat is always essential to the conversion of a solid into a solution. 

 Ordinarily the heat is abstracted from the surrounding media, and is 

 rendered latent in the solution , thus explaining the mode of action of 

 freezing mixtures. The amount of heat so rendered latent in forming a 

 solution is nearly always equivalent to the amount required to melt the 

 body. 



In certain cases, however, instead of the temperature being lowered 

 in the process of solution, it actually rises, as when caustic potash is dis- 

 solved in water. This depends upon the fact that two contrary proc- 

 esses are going on at the same time ; the solution, which tends always 

 to produce a reduction of temperature, and the chemical union of the 

 potash with the water, which, like many other chemical processes, tends to 

 cause an increase of temperature. Consequently, as one or the other of 

 these processes predominates the temperature will fall or rise ; or, if the 

 two balance, will remain unchanged. 



Solubility varies greatly in different bodies and in different liquids. 

 Some solids are soluble only in hot media, and are deposited on cool- 

 ing ; others only in cold liquids, and are thrown out of solution when 

 the temperature of the liquid is raised. As a rule, bodies dissolve in 

 liquids which have similar properties ; thus crystalline bodies are soluble 

 in water, fats in oil, metals in mercury, and resins in alcohol. 



" When two or more salts are dissolved in water without chemical 

 action on each other, three conditions result : 1st. The quantity of each 

 salt held in solution is less than when it alone is present, though the 

 combined quantity is greater than when only one salt is used. 2d. The 

 quantity of each is as great as when one only is used ; then the total 

 quantity dissolved is the sum of that taken up in each single solution. 

 3d. The quantity dissolved is greater than when one alone is used, 

 the addition of the second salt in this case increasing the solubility of 

 the first, and often the first increasing also the solubility of the second" 

 (Draper). 



When the cohesion of the solid and its adhesion for the liquid mole- 

 cules balance, the solution is then said to be saturated. In the case of 

 certain fluids, like alcohol and water, there is no limit to solubility ; their 

 molecules will freely mingle with each other, and the resulting liquid is 

 said to be a mixture, or an emulsion. On the other hand, two liquids 

 may offer an example of true solution, one being only capable of passing 

 to a certain degree between the molecules of the other, as in the case of 

 volatile oils and water, where the limit of solubility is readily reached. 



5. Imbibition.— Eyery porous solid may be considered as formed of 



