442 Dr, Thomson on [Dec. 



contraction; for if the anhydrous sulphate and the water, consti- 

 tuted the crystal of sulphate of soda united without any change 

 of volume, the specific gravity would be 1-75, instead of 1-35 ; 

 so that the specific gravity ot the water in the crystals is onlv 

 0-318. ^ ^ ^ -^ -y ^ 



If in the sesquisulphate we calculate the specific gravity of 

 the sulphuric acid on the supposition that it combines with the 

 anhydrous sulphate without any change of volume, we obtain 

 0-9. Now we are certain that the specific gravity of anhydrous 

 sulphuric acid ii at least 2. Calculated from the bisulphate, the 

 specific gravity of sulphuric acid would be 1*01. 



The specific gravity of sulphate of potash is 2*880 



' of bisulphate of potash 2*112 



If we calculate the specific gravity of the second atom of 

 sulphuric acid in the bisulphate, on the supposition that the 

 anhydrous sulphate, the sulphuric acid, and the water, unite 

 without any change of volume, we obtain 0923. 



Would it be premature to conclude from these facts that the 

 water of crystallization in neutral salts, and the excess of acid in 

 supersalts, undergo an increase of volume instead of a diminu- 

 tion? I could produce several additional examples of this 

 increase, if this were the proper place for entering upon such an 

 investigation. 



3. Prismatic Carbonate of Soda, 



I have been aware for some time that when the common octa- 

 hedral crystals of carbonate of soda are liquefied by heat in their 

 water of crystallization, and the solution set aside, new crystals 

 of carbonate of soda are formed, having a different shape, and 

 containing a smaller quantity of water. I have mentioned the 

 fact generally in p. 267, vol. ii. of my '' Attempt to establish the 

 First Principles of Chemistry by Experiment." But my experi- 

 ments had been made on too small a scale to enable me to deter- 

 mine the form of the crystals, or to subject the salt to an analysis 

 sufliiciently rigid to claim a place in my late work ; though I had 

 concluded from my trials that the water amounted either to seven 

 or eight atoms, I did not succeed in determining which. 



My friend Mr. Charles Tennant, of Glasgow, who manufac- 

 tures carbonate of soda on a very extensive scale, and who is in 

 the habit of conljinuing his processes during summer as well as 

 winter, found himself obliged to stop the crystallizing of the salt 

 during the very hot weather of the summer of 1825, which has 

 just finished. Before the stop took place, several crops of crys- 

 tals had been deposited in his evaporating pans quite different 

 in their appearance from the crystals of common carbonate of 

 soda. These crystals drew the attention of Mr. Thomas Clarke, 

 an exceedingly ingenious chemical friend of mine, who has the 



