jAmjAET 14, 1910] 



SCIENCE 



47 



homogeneous; and this dilution may be 

 carried on indefinitely. If, on the other 

 hand, we permit the blue liquid to evapo- 

 rate, we thus decompose it by abstracting 

 water from it. We say that the solution is 

 becoming more concentrated. This change 

 is a perfectly reversible one, and like all 

 chemical changes it follows the law of 

 mass action. The abstraction of water 

 from a solution of copper sulphate by 

 means of heat is just as truly an act of de- 

 composing that liquid as is the abstraction 

 of carbon dioxide from limestone when the 

 latter is heated. 



Blue vitriol is formed by the addition 

 of water to anhydrous copper sulphate. 

 The compound thus produced is quite 

 stable at room temperature. If now we 

 add anhydrous copper sulphate to crystals 

 of blue vitriol, the latter lose part of their 

 water content, which is taken up by the 

 anhydrous salt till equilibrium is estab- 

 lished. If, on the other hand, we treat the 

 blue vitriol crystals -with water, it is clear 

 that we can not thus dehydrate the crys- 

 tals. On the contrary, this added water 

 will, because of mass action, tend to in- 

 crease the stability of the complex which 

 we represent by the formula CuSO^.SHjO,, 

 and to this complex all of the additional 

 water present in the solution adds itself. 

 What then is the formula of the hydrate 

 contained in an aqueous copper sulphate 

 solution at known temperature? This 

 question is really an idle one, for since all 

 of the copper sulphate present is combined 

 with all of the water of the solution, the 

 composition of the hydrate is clearly ex- 

 pressed by CuS04.a;H20, where x repre- 

 sents the number of water molecules which 

 the entire solution contains per each 

 copper sulphate molecule; and so x 

 increases as we dilute the solution and 

 diminishes as we concentrate it. But this 

 must not be taken as meaning that all of 



the water in a copper sulphate solution is 

 equally strongly bound to the salt mole- 

 cules. Indeed, in the case under considera- 

 tion it is extremely probable that at least 

 five molecules of water are more strongly 

 bound to each copper sulphate molecule 

 in the solution, for as the salt separates 

 out, these five molecules remain in com- 

 bination as a part of the compound. But 

 while in the solution the copper sulphate 

 molecule plus five molecules of water may 

 be present as a nucleus to which the addi- 

 tional water molecules are attached, the 

 force of attraction with which the outly- 

 ing water molecules are held by the nucleus 

 shades off so gradually as the radius of the 

 sphere of influence increases that there is at 

 no point any very sharp demarcation, and 

 so it would be folly to attempt to ascribe 

 any definite formula whatever to the hy- 

 drate existing in the solution. Attempts to 

 deduce the formula of hydrates in solutions 

 from the boiling points or freezing points 

 of the latter are very far from the mark, 

 though to be sure boiling-point and freez- 

 ing-point curves do frequently show 

 maxima and minima which are doubtless 

 due to changes of intensity with which the 

 water and salt molecules are held together 

 as their relative number is changed. 

 Furthermore, it is very significant that 

 such maxima and minima in the boiling- 

 point and freezing-point curves are found 

 in the case of those substances, which, when 

 they crystallize from the solution, do so 

 with one or more molecules of the solvent 

 attached as so-called crystal water. It is 

 well known that at higher temperatures 

 salts separate from solutions with less crys- 

 tal water than at lower temperatures. In- 

 deed at high temperatures the anhydrous 

 salt is frequently in equilibrium with the 

 saturated solution. So while at ordinary 

 temperatures copper sulphate forms crys- 

 tals with five molecules of water, at lower 



