LECTURE I.] HISTORY OF CHEMISTRY. II 



the Egyptians in the manufacture of glass, 11 but Stahl was the 

 first to discover that common salt contained an alkali differing 

 from potash. 



Amongst the acids known at that period, I mention hydro- 

 chloric, nitric, sulphuric, and acetic. We are indebted to the 

 Arabian alchemists for the introduction of the use of aqua regia. 

 Scheele considerably increased the number of the organic 

 acids. He discovered hydrocyanic, malic, uric, lactic, citric, 

 oxalic, and gallic acids. The discovery of hydrofluoric acid 

 also stands to his credit. So we see what a large number of 

 salts the phlogiston period had at its command in consequence 

 of these discoveries. I do not enlarge upon this, but turn to 

 the knowledge respecting the gases, which are all the more 

 interesting because they led to the downfall of the phlogiston 

 theory. 



All gases were for a long time regarded as identical with 

 air, and this in turn was considered to be an element. Van 

 Helmont, in the middle of the seventeenth century, was the 

 first who assumed the existence of different gases. Nearly 

 another hundred years passed after this assumption before the 

 recognition of a gas which was certainly different from air ; the 

 difficulties of the manipulation make this easily comprehensible. 

 We are especially indebted to Black, Cavendish, and Priestley 

 for surmounting these difficulties. The first examined carbonic- 

 anhydride, or the so-called fixed air, and corrected the views 

 as to mild and caustic alkalies. His investigation 12 is one of 

 the most important of the phlogiston period. In it (as was 

 done by Lavoisier at a later date) we find the relations by 

 weight brought forward as the most important consideration in 

 the arguments. Cavendish studied the properties of hydrogen, 

 whilst Priestley discovered oxygen, nitric oxide, and carbonic 

 oxide, as well as sulphurous and hydrochloric acid gases, 

 ammonia, and silicon fluoride. 



11 Kopp, Geschichte. 4, 27. 



12 Experiments upon Magnesia Alba, Quicklime and some other 

 Alcaline Substances. See Alembic Club Reprints, No. I. 



