460 KEPORT— 1886. 



(MgS047H20 

 1 CaS042H20 



lBaS04 



CaCl26H20 /Mg(N03)26HoO 



SrCl^BH,© Ca(N03)26H2b 



BaCl22H20 J Sr(N03)24H20 



.PbCl2 1 or anhydrous from hot solution. 



Ba(N03)2 



VPb(N03)2 



In the preceding examples disposition to combine with water seems 

 to depend more on the nature of the basic radicle than on the acid radicle 

 of the salt. On the other hand we have 



NaCl 



NaNOs ; bnt NaaCOslOHaO 



Na2SO4l0H2O 

 Na2HP04l2H20, &c. 



The fusibility of a substance has much to do with its solubility. This 

 has been pointed out by Carnelley (' Phil. Mag.' March 1882) in reference 

 to carbon compounds, and by myself and Mr. Shenstone (' Phil. Trans.,' 

 1884, and ' Jour. Chem. Soc' July 1884) in reference to salts. 



Neither fusibility alone nor chemical constitution alone seems to be 

 sufficient to determine whether a solid shall be soluble or not ; but it may 

 be taken as a rule to which there are no exceptions that when there is 

 close connection in chemical constitution between a liquid and solid, and 

 the solid is at the same time easily fusible, it will also be easily soluble in 

 that liquid.' 



I take it that a salt containing water of crystallisation may be con- 

 sidered as closely resembling water itself. For example — 



MgS04 7H20 



may be considered as a congeries of eight molecules of water, H2O7H2O, 

 in which one molecule of water is replaced by the elements of the salt. We 

 know that exchange of a similar kind is possible in the case of water of 

 Lalhydration, as long ago pointed out by Graham. 



MgS04H206H20 [giving MgS04K2S046H20, &c. 



So that MgS04 7H20 being like solid water in constitution, and also 

 easily fusible by heat (it melts at 70°), it is easily soluble in water, and 

 the solubility increases rapidly with rise of temperature. 



The effect of fusibility on increase of solubility with rise of tempera- 

 ture is well shown by comparison of two such salts as 



Potassium chlorate . . . m.p. 359° 

 Potassium chloride . . . m.p. 734° 



Ijoth of which are destitute of water, and are therefore comparable (see 

 fig. 5, p. 46). 



There are many substances which contain much water — or the elements 

 of water — which, nevertheless, do not dissolve in water, or dissolve with 



' This, of course, does not explain how it is that, although silver chloride is 

 insoluble in water, the less fusible sodium chloride is easily soluble. 



