96 PRINCIPLES OF CHEMISTRY 



of water per 142 parts of anhydrous salt, and not 180 parts of water, as 

 in the above-mentioned salt. Further, the crystals containing TH 2 O 

 are distinguished for their instability ; if they stand in contact not only 

 with crystals of Na 2 SO 4 ,10H 2 O, but with many other substances, they 

 immediately become opaque, forming a mixture of anhydrous and deca- 

 hydrated salts. It is evident that between water and a soluble sub- 

 stance there may be established different kinds of greater or less stable 

 equilibrium, of which solutions form one aspect/' 7 



and one must think that the connection with the fusibility of the deca-hydrated salt 

 (and of all salts which easily give supersaturated solutions and are capable of forming 

 several crystallohydrates), and with that decomposition (formation of the anhydrous 

 salt) which the deca-hydrated salt suffers on melting plays its part here. As some 

 crystallohydrates of salts (alums, sugar of lead, calcium chloride) melt without 

 decomposing, whilst others (like Na 2 SO 4 ,H. 2 O) are decomposed, then it may be that the 

 latter are only in a state of equilibrium at a higher temperature than their melting point. 

 Did experiment show that the hepta-hydrated salt began to crystallise below 33, and 

 that then only the crystals grow, then all the data concerning supersaturated solutions of 

 sodium sulphate could be explained exclusively in the sense of a super-cooling effect. 

 At present, however, these questions, notwithstanding the mass of research to which 

 they have been subjected, cannot be considered as fully resolved. It may here be 

 observed that in melting crystals of the deca-hydrated salt, there is formed, besides 

 the solid anhydrous salt, a saturated solution giving the hepta-hydrated salt, so that this 

 passage from the deca- to the hepta-hydrated salt, and the reverse, takes place with the 

 formation of the anhydrous (or it may be, mono-hydra ted) salt. 



The researches of Pickering (1887) on the amount of heat which is evolved in the 

 solution of hydrous and anhydrous salts at different temperatures, give reason to think 

 that at a certain temperature no heat will be evolved in the combination with water; that 

 is, that probably such a combination will not take place. Thus 106 grams (the molecular 

 weight in grams) of anhydrous sodium carbonate, NaoCOj, in dissolving in 7,200 grams 

 ( = 400 H 2 O) of water, evolve 4,300 calories at 4, 5,300 at 16, and 5,850 calories at 25 (in 

 other cases the heat evolved in solution also increases with a rise of temperature). If, 

 however, the crystallo- hydrate, NaoCO^ , 10H. 2 O,be taken, then (for the same quantity of 

 anhydrous salt) an absorption of heat is observed; at 4 -16,250, at 16 16,150, and at 

 25 16,300 calories. As in this case a portion of the heat absorbed is due to the fact that 

 the water of crystallisation taken in a solid state appears in a liquid state, Pickering sub- 

 tracts the latent heat of liquefaction of ice, and obtains in the given case at 4 -1,700, at 

 16 600, and at 28 -0 calories. From this, the heat of the formation of the crystallo- 

 hydrate, or the heat evolved by the combination of Na 2 CO 3 with 10H 2 O, may be 

 calculated (by subtracting the former quantities from the first). At 4 it is equal to 

 + 6,000, at 16 + 5,900, at 25 + 5,850 calories; that is, it distinctly decreases, although 

 but slightly, with the rise of temperature. It may be that for Na 2 SO 4 at 33 the heats 

 of the formation of + lOHoO and 7H 2 O differ but very slightly. 



57 Emulsions, like milk, are composed of a solution of glutinous or like substances, 

 or of oily liquids suspended in a liquid in the form of drops, which arc clearly visible 

 under a microscope, and form an example of a mechanical formation which resembles 

 solutions. But the difference from solutions is here evident. There are, however, 

 solutions which approach very near to emulsions in the facility with which the substance 

 dissolved separates from them. It has long been known, for example, that a particular 

 kind of Prussian blue, KFe 2 (CN) 6 , dissolves in pure water, but, on the addition of the 

 smallest quantity of either of a number of salts, it curdles and becomes quite insoluble. 

 If copper sulphide (CuS), cadmium sulphide (CdS), arsenic sulphide (As 2 S-), and many 

 other metallic sulphides, be obtained by a method of double decomposition (by precipi- 



