864 PEINCIPLES OF CHEMISTRY 



temperature on shaking, the whole of the cobalt will be separated 

 in the form of black cobaltic oxide : 2CoS0 4 + C1HO + 2BaC0 3 



boils at 13 (aldehyde, its isomeride, boils at 21) ; therefore the product disengaged by 

 the splitting up of the hydrate boils at 184 lower than the hydrate C 2 H 4 (HO) 2 . Thus 

 the hydrate C 2 H 3 (HO) 3 , which ought to boil at about 800, splits up in exactly the same 

 way into water and the product C 2 HjO 2 , which boils at 117 that is, nearly 183 lower 

 than the hydrate, C 2 H 3 (HO) 3 . But this hydrate splits up before distillation. The 

 above-mentioned hydrate of acetic acid .is such a decomposable hydrate that is to 

 say, what is called a solution. Still less stability may be expected from the following 

 hydrates. C 2 H 2 (HO) 4 also splits up into water and a hydrate (it contains two hydroxyl 

 groups) called glycollic acid, C 2 H 2 O(HO) 2 = C 2 H 4 O 3 . The next product of substitution 

 will be 2 H(HO) 5 ; it splits up into water, H 2 O, and glyoxylio acid, C 2 H 4 O 4 (three 

 hydroxyl groups). The last hydrate which ought to be obtained from C 2 H 6 , and ought 

 to contain C 2 (HO) 6 , is the crystalline compound of oxalic acid, C 2 H 2 O 4 (two hydroxyl 

 groups), and water, 2H 2 0, which has been already mentioned. The hydrate C 2 (HO)a 

 = C 2 H 2 O 4 ,2H 2 O, ought, according to the foregoing reasoning, to boil at about 600 

 (because the hydrate, C 2 H 4 (HO) 2 , boils at about 200, and the substitution of 4 hydroxyl 

 groups for 4 atoms of hydrogen will raise the boiling-point 400). It does not resist this 

 temperature, but at a much lower point splits up into water, 2H 2 0, and the hydrate 

 C 2 O 2 (HO) 2 , which is also capable of yielding water. Without going into further dis- 

 cussion of this subject, it may be observed that the formation of the hydrates, or com- 

 pounds with water of crystallisation, of acetic and oxalic acids has thus received aa 

 accurate explanation, illustrating the point we desired to prove in affirming that com- 

 pounds with water of crystallisation are held together by the same forces as those which 

 act in the formation of other complex substances, and that the easy displaceability 

 of the water of crystallisation is only a peculiarity of a local character, and not 

 a radical point of distinction. All the above-mentioned hydrates, C 2 X<3, or pro- 

 ducts of their destruction, are actually obtained by the oxidation of the first hydrate, 

 C 2 H 5 (HO), or common alcohol, by nitric acid (Sokoloft and others). Hence the forces 

 which induce salts to combine with nH 2 O or with NH 3 are undoubtedly of the same 

 order as the forces which govern the formation of ordinary ' atomic ' and saline com- 

 pounds. (A great impediment in the study of the former was caused by the conviction 

 which reigned in the sixties and seventies, that 'atomic' were essentially different 

 from 'molecular* compounds like crystallohydrates, in which it was assumed that 

 there was a combination of entire molecules, as though without the participation of the 

 atomic forces,) If the bond between chlorine and different metals is not equally strong, 

 eo also the bond uniting ?iH 2 O and NH 3 is exceeding variable; there is nothing very 

 surprising in this. And in the fact that the combination of different amounts of NH$ 

 and H 2 alters the capacity of the haloids X of the salts RX 2 for reaction (for instance, 

 in the luteo-salts all the X 3 , while in the purpureo, only 2 out of the 3, and in the prazeo- 

 salts only 1 of the 8 X's reacts), we should see in the first place a phenomenon similar 

 to what we met with in Cr 2 Cl e (Chapter XXI., Note 7 bis), for in both instances the essence 

 of the difference lies in the removal of water; a molecule RC1 3 ,6H 2 O or RCl^GNHj 

 contains the halogen in a perfectly mobile (ionised) state, while in the molecule 

 HC1 3) 5H 2 O or RC1 3 ,5NH 3 a portion of the halogen has almost lost its faculty for reacting 

 with AgNO 3 , just as metalepsical chlorine has lost this faculty which is fully developed in 

 the chloranhydride. tlntil the reason of this difference be clear, we cannot expect that 

 ordinary points of view and generalisation can give a clear answer. However, we may 

 assume that here the explanation lies in the nature and kind of motion of the-atoms in tb.6 

 molecules, although as yet it is not clear how. Nevertheless, I think it well to call 

 attention again (Chapter I.) to the fact that the combination of water, and hence, also, 

 Of any other element, leads to most diverse consequences ; the water in the gelatinous 

 hydrate of alumina or in the decahydrated Glauber salt is very mobile, and easily reacts 

 like water in a free state ; but the same water combined with oxide of calcium, or C 2 H4 

 (for instance, in C 2 H 6 and in C 4 H 10 O), or with P 2 5 , has become quite different, and no 



