A Sketch of the Laws of Chemical Combination, 319 



centrated sulphuric acid is a compound of real acid and one equivalent of 

 basic water, which cannot be separated without great difficulty. 



From these facts it is evident that a certain part of the water, entering 

 into the constitution of salts formed by these acids, must be considered as 

 basic ; whilst the rest is merely necessary to enable the salt to assume its 

 crystalline form. Thus crystallized sulphate of zinc contains seven equi- 

 valents of water, of which six may be easily driven off, but one equivalent 

 remains in combination at 410° and all inferior temperatures. This salt 

 may therefore be considered as a double sulphate of oxide of zinc and 

 water, with six equivalents of water of crystallization. The atom of basic 

 water may be replaced by one of sulphate of potassa, so as to form a true 

 double salt, without any change in external form ; and this fact appears to 

 form a clue to the tendency of this sulphate, and others of similar consti- 

 tution, to form double sulphates. Again, plaster of Paris appears to 

 consist of a sulphate of lime, and two equivalents of basic water. At a 

 temperature not exceeding 270o this salt loses its water, but retains the 

 power of recombiuing with it, or setting ; but if heated above 300° it 

 becomes properly sulphate of lime, and loses the tendency to recombine 

 with water, — a change perfectly analogous with that of a phosphate to a 

 paraphosphate. 



It is in the constitution of the salts of phosphoric and arsenic acid, 

 which are isomorphous, tliat the most striking facts of this kind may be 

 observed. As was just now stated, these acids have a tendency to com- 

 bine with three equivalents of water, or of an alkaline base, so as to form 

 salts. By substituting one atom of soda for one of basic water, we obtain 

 a salt called the acid triphosphate of soda, as the acid is not neutralized 

 by the alkali. If another atom of water be exchanged for one of soda, a 

 neutral triphosphate is obtained, which is the most frequent combination ; 

 and if the whole of the water be replaced by soda, an alkaline triphosphate 

 is produced. Both tlie acid and neutral triphosphates may be converted 

 into paraphosphates by heat; and double salts may be easily formed by 

 substituting one base for part of the water, and another base for the rest. 



All these salts require a certain quantity of water of crystallization, to 

 enable tliera to assume tiieir regular forms; and it is remarkable that the 

 neutral triphosphate usually crystallizes in one form — the common rhombic 

 phosphate — with 24 equiv. of water ; but that when its solution is eva- 

 porated at a temperature of 90°, a different set of crystals is produced, 

 having only 14 equiv. of water. These are the two salts to which Sir J. 

 Hcrschel refers, when speaking of the laws of isomorphism (Prelim. Disc. 

 |). 170), and combinations precisely analogous arc formed by arsenic acid. 



Tlie (piantity of combined water is very variable in different salts, but 

 uniform in the same, except in cases analogous to the one just mentioned. 

 A salt may contain double its own weight of water, and yet be perfectly 

 dry ; but on exposure to iicat, it is dissolved in its own water, undergoing 



