XVI PREFACE. 



believe to be that the sulphate of zinc which I used 

 was a pure salt, while zinc in the metallic state is 

 never quite free from iron and some other impurities, all of 

 which have a tendency to diminish its apparent atomic 

 weight. 



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As the analysis of sulphate of zinc constitutes in some 

 measure the foundation of the whole superstructure which 

 I have endeavoured to rear, I made the analysis of it with 

 the greatest possible care. My first analysis of it was 

 inserted in the Annals of Philosophy, (second series) I. 

 247. But by some inadvertency, the quantity of water is 

 stated at six atoms instead of seven, which is the true 

 quantity. I cannot very well account for this mistake; 

 for on looking back to my original analysis, I find it 

 merely stated that 17 grains of sulphate of zinc were just 

 decomposed by 13*25 grains of chloride of barium. No- 

 thing is said about the state of the salt, which would 

 enable us to account for the absence of an atom of water. 

 But I have repeated the analysis of sulphate of zinc 

 many times since the publication of that paper, and have 

 made several of my practical students repeat it, and the 

 uniform result has been seven atoms of water, and not six, 

 MS I there stated. If solutions of 17 grains of sulphate 

 of zinc and 13*25 of chloride of barium be mixed, we shall 

 find that the supernatant liquid, after the deposition of the 

 sulphate of barytes, will become milky when mixed with 

 sulphate of soda, showing that it still retains barytes, 



