110 PRINCIPLES OF CIIE.M: 



or sulphuric acid, is such a compound. If a larger proportion of water 

 be taken, it will combine with the H 2 SO, ; for instance, if M parts of 

 water per 80 parts of sulphuric anhydride be taken, a compound is 

 formed which crystallises in the cold, and melts at -+- 8, whilst oil of vitriol 

 does not solidify at even 30. If still more water be taken, the oil of 

 vitriol will dissolve in the remaining quantity of water. An evolution 

 of heat takes place, not only on the addition of the water of constitu- 

 tion, but in a less degree on further additions of water. 72 And 

 therefore there is no distinct boundary, but only a gradual transition, 

 between those chemical phenomena which are expressed in the forma- 

 tion of solutions and those which take place in the formation of the 

 most stable hydrates. 73 



72 The diagram given in note 28 shows the evolution of heat <m the mixture of 

 sulphuric acid, or mono-hydrate (HoSO 4 , i.e. SOs + H-jO), with different quantities of uat ri- 

 per 100 vols. of the resultant solution. Per 98 grams of sulphuric acid iH..SO ( l there are 

 evolved, on the addition of 18 grams of water, 6,379 units of heat ; with <l<ml>le or three 

 times the quantity of water 9,418 and 11,187 units of heat, and with an infinitely large 

 quantity of water 17,860 units of heat, according to the determinations of Thomsen. He 

 also showed that when HoSO 4 is formed from SO 3 ( = 80) and H. 2 O ( = ]KI. 21. ms units of 

 heat are evolved per 98 parts by weight of the resultant sulphuric acid. 



" Thus, for different hydrates the stability with which they hold water is very dis- 

 similar. Certain hydrates hold water very loosely, and in combining with it evolve 

 little heat. From other hydrates the water cannot be separated by any degree of heat, 

 even if they are formed from anhydrides (i.e., anhydrous substances) and water with 

 little evolution of heat; for instance, acetic anhydride in combining with water evolves an 

 inconsiderable amount of heat, but the water cannot then be expelled from it. If the 

 hydrate (acetic acid) formed by this combination be strongly heated it either volatilises 

 Avithout change, or decomposes into new substances, buj> it does not again yield the original 

 substances i.e., the anhydride and water. Here is an instance which gives the reason 

 for calling the water entering into the composition of the hydrate, water of constitution. 

 Such, for example, is the water entering into the so-called caustic soda or sodium 

 hydroxide (see note 71). But there are hydrates which easily part with their water; yet 

 this water cannot be considered as water of crystallisation, not only because sometimes 

 such hydrates have no crystalline form, but also because, in perfectly analogous cases, 

 very stable hydrates are formed, which are capable of particular kinds of chemical 

 reactions, as we shall learn afterwards. In a word, there is not a distinct boundary 

 either between the water of hydrates and of crystallisation, or between solution and 

 hydration. 



It must be observed that in separating from an aqueous solution, many substances, 

 without having a crystalline form, hold water in the same unstable state as in crystals ; 

 only this water cannot be termed 'water of crystallisation' if the substance which 

 separates out has no crystalline form. The hydrates of alumina and silica are examples 

 of such unstable hydrates. If these substances are separated from an aqueous solu- 

 tion by a chemical process, then they always contain water, and when dried at a 

 definite temperature, so that the hvgroscopic water may pass off, these substances hold 

 water in a definite proportion. The formation of a new chemical compound containing 

 water is here particularly evident, for alumina and silica in an anhydrous stat have 

 properties differing from those they show when combined with water, and do not combine 

 directly with it. The entire series of colloids on separating from water form similar 

 compounds with it, which have the aspect of solid substances generally, without crystal- 

 line structure. Besides which, colloids retain water in other different states (srr notes :>7 



