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 anticipated 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 experiments with "black magnesia," or the binoxide of manganese 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 discoverer 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 investigations on light, led to the discovery of hydrofluoric acid, and its property of acting on glass. In the course of three years' researches
