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ANNALS 



OP 



PHILOSOPHY 



MARCH, 1822. 



Article I. 



Experiments to determine the Weight of an Atom of Alumina. 

 By Thomas Thomson, MD. FitS. Regius Professor of Che- 

 mistry in the University of Glasgow. 



It is not unUkely that the labour which I have bestowed in 

 order to render the following experiments as accurate as, 

 possible will appear to some persons a waste of time. But I 

 am of opinion that it is of the greatest importance to determine 

 the atomic weights of bodies with the utmost possible precision. 

 When this desirable object is gained, the art of analysis, at pre- 

 sent so laborious and so uncertain, will be greatly simplified. 

 Besides, alumina being a constituent so generally found in crys- 

 tallized minerals, an exact knowledge of its atomic weight 

 cannot but throw considerable light upon the constitution of a 

 very numerous and interesting series of crystalhzed minerals. 



I have examined a considerable number of the salts of 

 alumina, but found none of them fit for my purpose, except 

 common alum — a salt which crystallizes with great ease and 

 regularity, and which can be readily obtained in a state of the 

 most perfect purity. It is not sensibly altered by exposure to 

 the air, and may, therefore, be obtained without difficulty at all 

 times in the same state. It is well known, that alum consists of four 

 different constituents, which are always present in it in exactly 

 the same proportion ; namely, sulphuric acid, alumina, potash, 

 and water. The object of my experiments was to determine the 

 weight of each of these constituents in 100 grains of alum, 

 crystals. 



1 . I may observe, before detaihng my own experiments, that 

 'New Series, vol. iii. m 



