HISTORICAL INTRODUCTION. 3 



potash. Of course, his results convey no idea 

 whatever of the true composition of the salts which 

 he attempted to analyze. We might, indeed, by 

 subtracting the carbonic acid from the potash, 

 and adding it to the acid, deduce the composi- 

 tion of these salts according to his experiments. 

 These deductions being made, it will be found 

 that saltpetre according to his experiments, is 

 composed of 



Potash 6 , 



Nitric acid 6-68 



12-68 



Numbers which, though not correct, constitute 

 a pretty good approximation, and come much 

 nearer the truth than the analysis of Kirwan, or 

 even the more careful analyses of Berard or 

 Wenzel. 



The experiments of Plummer were made in 

 the same way, and were subject to the same 

 error. 



In Dr. Black's celebrated Essay on Magnesia 

 and Quicklime, he first pointed out the mistake 

 of Homberg, and from some of his experiments 

 deductions might be made respecting the con- 

 stituents of some of the magnesian salts; but not 

 being professed analyses, they have not attracted 

 much of the attention of succeeding chemists. 



Soon after the middle of the eighteenth cen- 

 tury, attempts were made by different chemists 



A2 



