842 THE POPULAR SCIENCE MONTHLY. 



on Air and Fire " was written in 1775, and published in German in 

 1777 ; was translated into English in 1780, and into French in 1781. 

 It contained many facts of great value, together with theories on the 

 nature of combustion, fire, light, and heat, which can have now only- 

 historical importance. His researches on the subject, although they 

 were parallel with those of Priestley, and although Priestley antici- 

 pated him in the discovery of oxygen, were conducted without any 

 knowledge of what the English chemist was doing. Scheele showed 

 in his treatise, from numerous simple and ingenious experiments, that 

 air is composed of two gases, in a proportion, as he calculated, of about 

 three to one ; and he described the special properties of oxygen and 

 nitrogen, as we know them, with their effects on combustion and on 

 animal and plant life. 



From oicoeriments with " black magnesia," or the binoxide of man- 

 ganese and saltpeter, now familiar to all students, Scheele deduced 

 the theory that heat was a combination of phlogiston and oxygen, 

 while combustion was the combination of the oxygen of the air with 

 the phlogiston of the combustible body, resulting in the formation of 

 the compound above named, or heat. Light was also a combination 

 of oxygen and phlogiston, but richer in phlogiston than heat. The 

 different kinds of light were different combinations of oxygen and 

 phlogiston an assertion which was based upon the fact that violet 

 light exercises a stronger decomposing influence on the chloride of 

 silver than does light of the other colors. Thus Scheele was the dis- 

 coverer of the fact which is the basis of photography. He found that 

 fluor-spar became phosphorescent when heated moderately, but not 

 when heated to incandescence. This was because the mineral contains 

 phlogiston, which, under a moderate heating, unites with the heat and 

 forms light ; but when heat is applied to the degree of incandescence, 

 the phlogiston is all taken away and light can not be formed. 



Hydrogen, or inflammable air, as it was then called, he regarded as 

 composed of phlogiston and heat. But after Lavoisier, Cavendish, 

 and Priestley had shown that water is produced by the combustion of 

 hydrogen, and hydrogen is formed by passing the vapor of water over 

 incandescent iron, Scheele changed his theory of oxygen, and assumed 

 that it was composed of a saline principle of water and phlogiston ; 

 of these components, the former gave heat with phlogiston, and water 

 caused an increase in the weight of the burned body. These theories 

 attracted much attention at the time ; but they are no part of science, 

 for they were quickly dispelled by the publication of Lavoisier's more 

 correct views on combustion. But the facts which Scheele sought to 

 explain by them nearly all his own discoveries remain, valuable 

 gifts to chemistry. 



His experiments with fluor-spar, carried on in the course of his in- 

 vestigations on light, led to the discovery of hydrofluoric acid, and its 

 property of acting on glass. In the course of three years' researches 



