1 822.] Weight of an Atom of Alumina. ] ^ 



Sulphuric acid 32*8542 



Alumina 11-0682 



Potash 9-8562 



Water 46-2012 



99^998 ' 



Or it may be stated in this way : 



Sulphate of alumina 35-72885 



Sulphate of potash 18-06975 



Water 46-20123 



99^99983 



But it is much more convenient in general, because we are net 

 perplexed by a great number of decimal places, to employ in our 

 calculations the weight of an atom of the salt. The atomic 

 weight of an integral particle of any salt never can contain more 

 tlian three decimal places. When the atoms of water in it are 

 represented by an even number, then the decimal places never 

 can exceed two. 



It has been alleged that alum owes its property of reddening 

 vegetable blues to a quantity of bisulphate of potash which it 

 contains ; and this opinion has been supported by the following 

 experiment : Mix together solutions of sulphate of alumina and 

 sulphate of potash — a precipitate, it is said, appears. Hence it 

 is alleged that the sulphate of potash is converted into bisul- 

 phate of potash, and that the alumina thus partly deprived qf 

 acid becomes insoluble, and occasions the precipitate. I bav^ 

 repeated this experiment with all possible care, and with salts in 

 a state of purity. I never could obtain any immediate precipi^ 

 tate whatever ; but when the mixed liquid was allowed to remain 

 for 24 hours, there was always a deposit of alum crystals. We 

 have, therefore, no evidence whatever of the presence of bi&ul- 

 phate of potash in alum ; and the preceding experiments are 

 quite incompatible with such a supposition. 



Article II. 



On certain Saline Solutions which man be cooled without Crys- 

 taliizatiou ; hut deposit Crystals when agitated. By Thomas 

 Thomson, MD. FRS. Regms Professor of Chemistry in the 

 University of Glasgow. 



It has been long known to chemists that a saturated solution 

 of sulphate of soda in a well corked phial may be cooled down 

 to the common temperature of the atmosphere without the depo- 



