24 Sketch of ihe latest Improvements [Jan, 



mixed together in unknown proportions, ferrureted chva^ate of 

 potash cannot be employed in such cases till a good niethod is dis- 

 covered ot separating manganese froii) iron. Gehlen's rnetUod i$ 

 the best hitherto proposed ; tiiough it only answers when the whole 

 of the iron is in the state of peroxide. 



I iri^y rnejuion here, for the sake of pur British maiiufacturers of 

 sulphuric acid, that E. W. Martius announced in llBU, it» a 

 German journal, that he had found white arsenic in a giass carboy 

 of English sulphuric acid. The arsenic had sep^rsjtied from the 

 acid, and formed a crust in the inside pi the glasij. As the notice 

 merely states the fact cf the separation of white arsenic , I conceive 

 it possible that Marti ir-^ might have been mistaken. If his state- 

 ment be accurate, the arsenic roust have made its way into the 

 carboy by some odd accident or other; for I think it hardly possible 

 that it should have been mixed with the sulphur before its com- 

 bustion. 



VI. Acids. 



1 . Formic Acid. — The reader is probably aware that this acid, 

 which exists ready formed in red ants, was originally discovered l)y 

 Ray and Fisher; that it was first accurately examined by Margraat; 

 that Arvidson and Oern published a very con)i)lete set of experi- 

 ments on it in 1777- It continued to be considered as a peculiar 

 acid till Fourcroy and Vauquelin published a set of experiments on 

 it in 1803, and drew as a conclusion from them that it is not a 

 peculiar acid, but a mixture of the acetic and malic acids. This 

 dissertation convinced all the French chemists ; and induced me in 

 the ^rcond edition of my System of Chemistry to expunge it from 

 the list of acids. But Suerzon having published a new set of expe? 

 riments on it, in 1805, showed that the premises of the French 

 chemists were insufficient to warrant their conclusions, and that 

 formic acid possessed striking and essential properties which distin^ 

 guished it from acetic acid. I suggested in my System the propriety 

 of a more rigid examination of the formates than had hitherto 

 taken place. This seems to have induced Gehlen to undertake a 

 laborious set of experiments on the subject. He observes in the 

 outset, that if 1 had been acquainted with the experiments of 

 Arvidson and Oern, and of Richter, on ilie formates, I should 

 ])robab]y have been satisfied, witliout requiring any farther proofs, 

 1 regret that it has never been in my power to peruse either of the 

 works alluded to by Gehlen. My knowledge of the experiments of 

 Arvidson and Oern is derived from the account of them given by 

 Keir in his Dictionary, and by Bergman in his Treatise on Elective 

 Attractions, Gehlen's experiments are quite decisive; but they are, 

 unfortunately, too long to be detailed here. He prepared, in the 

 first place, formate of copper, from which he separated the formic 

 acid by distilling it witli sulphuric acid in a retort. He compared 

 the pure formic acid thus obtained with acetic acid, procured from 

 acetate of lead by a similar process. 



