IS 14.] Mr. Scheele. 165 



baffled the sagacity of Margraaf, who had published a dissertation 

 on it some years before. Scheele. showed that it could be resolved 

 into lime and a peculiar acid called fluoric acid, the singular pro- 

 perties of which he ascertained and described. In this dissertation 

 he fell into a curious mistake. He thought that silica was a com- 

 pound of water and fluoric acid. But this mistake was speedily 

 rectified and acknowledged by Scheele himself, who soon found 

 that the silica which appeared in his experiments was derived from 

 the glass vessels corroded by the fluoric acid. Excepting the experi- 

 ment of Dr. Priestley, little was added to the facts ascertained by 

 Scheele respecting fluoric acid. But within these few years, many 

 new and important facts have been ascertained by the happy 

 sa acity of Gay-Lussac, Thenard, and Davy ; and a new view has 

 been taken of the subject, concerning the accuracy of which 

 chemists have not yet given a decided opinion. 



3. Sehcele's dissertation on manganese was published in 177*, 

 while he was at Upsala; and I have been told that his experiments 

 had been undertaken at the request of Bergman. I consider it as 

 the most important of all his chemical labours. It contains four 

 discoveries, any one of which would have been sufficient to secure 

 the reputation of the discoverer: — 1. That black oxide of manga- 

 nese is a substance different from every other, and lias a strong 

 tendency to unite with phlogiston ; in other respects agreeing with 

 the metallic earths or calces. Hence the inference was easy, and 

 unavoidable, that it is of a metallic nature. Accordingly this 

 inference was immediately drawn by Bergman. 2. That when 

 muriatic acid and black oxide of manganese are heated together, an 

 effervescence takes place, and a gaseous matter passes over, having 

 a yellow colour, and very remarkable properties. This substance 

 lie considered as muriatic acid deprived of phlogiston. It was 

 afterwards called oxymuriatic acid : and Davy, who has lately 

 revived the theory of Scheele, has given it the name of chlorine. 

 3. That there is occasionally mixed with black oxide of manganese 

 an earth hitherto unknown, but the most remarkable properties of 

 which he describes. This is the earth now well known by the name 

 <>f barytes. 4. That when ammonia is digested upon nitrate of 

 manganese it is totally decomposed. Its phlogiston combines with 

 the manganese, while its other constituent makes its escape in the 

 form of gas. Scheele had, no doubt, examined this gas, since he 

 informs us that it possessed properties very different from carbonic 

 acid gas ; but he gives no description of it. It is now well known 

 to be a/otic gas. Hence as this dissertation was published in 177-', 

 and was the result of three years' labour, there can be scarcely any 

 doubt that Scheele was in fact the first discoverer o[ azotic gas. 



4. His next paper, published in 1775, was on a new Method of 

 obtaining Benzoic Acid from Benzoin. It consists in boiling pow- 

 dered benzdta with quick-lime and water, filtering the solution, 

 evaporating it till nifficiently concentrated, and then precipitating 

 the acid by means of muriatic acid. 



