26. Prof. Graham on the Water of 



by concise, general, and undeniable reasoning, that my friend 

 niiwht have advantageously spared himself the trouble of 

 offering his numerous observations, had he paid more regard 

 to the logical strictness of his assumptions. 

 London, June 4, 1836. 



VII. On the Water of Crystallization of Soda-alum. By 

 Thomas Graham, F.R.S.E., Professor of Chemistry in the 

 Andersonian University, Glasgow; Corr. Member of the 

 Royal Academy of Sciences of Berlin, 8fc.* 



I^HE double sulphate of alumina and soda crystallizes in 

 the form of the regular octohedron, like the sulphate of 

 alumina and potash, while the former salt is supposed to con- 

 tain twenty-six atoms of water, and the latter contains only 

 twenty-four. The coincidence in form of these two salts is 

 most interesting, for in no other corresponding salts of potash 

 and soda has such a relation been observed from which any 

 inference in respect to isomorphism could properly be drawn. 

 Yet if the soda-salt contains two atoms more of water than the 

 potash-salt, the conclusion which follows is, not that soda and 

 potash are isomorphous bodies, but that soda plus two atoms 

 water is isomorphous with potash, as ammonia plus one atom 

 of water is isomorphous with the same body. But the last 

 analogy is superficial and likely to prove illusory. 



The exact determination of the water of crystallization of a 

 salt is often a problem of no inconsiderable difficulty, as many 

 precautions must be taken which are by no means obvious. 

 To have alumina free from potash or ammonia, it was pre- 

 cipitated from pure potash-alum by means of carbonate of 

 soda. A solution of sulphate of alumina was formed by dis- 

 solving the precipitated alumina in the proper quantity of sul- 

 phuric acid, and the requisite proportion of sulphate of soda 

 was added. A considerable crop of crystals of soda-alum 

 were obtained from the above solution allowed to evaporate 

 spontaneously in air. 



Like most very soluble salts the crystals of soda-alum, 

 when newly prepared, retain hygrometrically a notable quan- 

 tity of the saline liquor in which they have been formed. But 

 the crystals of this salt cannot be dried easily, as after they 

 lose their hygrometric water they are nearly as efflorescent as 

 sulphate of soda itself. Before being submitted to analysis 

 the crystals had been kept for five months of cold weather in 

 a large phial stopt by a cork. Their surfaces remained per- 



* Communicated by the Author. 



