From his letters and from the verdict of his contemporaries, Scheele is depicted as an amiable and honourable man, singularly free from vanity and selfishness. Unfortunately no portrait of him has survived. His last memoir on the action of sunlight on nitric acid was published in 1786; he died suddenly at the early age of forty-three in May of that year, two days after his marriage to Sara Margaretha Pohl. His devotion to science had told on his health, and his death was caused by a complication of diseases. Yet he was during his life, as after his death, regarded as one of the greatest of chemists: his great knowledge, extraordinary aptitude in experimenting, and high intellectual powers placed him among the foremost men of science of his day. Near the beginning of his _Treatise on Air and Fire_,[10] Scheele defines air. It is that fluid invisible substance which we continually breathe; which surrounds the whole surface of the earth, is very elastic, and possesses weight. “It is always filled with an astonishing quantity of all kinds of exhalations, which are so finely divided in it that they are scarcely visible, even in the sun’s rays.”[11] It also contains another elastic substance resembling air, termed aerial acid by Bergman (identical with Black’s fixed air). Since atmospheric air has not been completely converted into fixed air by admixture of foreign materials, “I hope I do not err if I assume as many kinds of air as experiment reveals to me. For when I have collected an elastic fluid, and observe concerning it that its expansive power is increased by heat and diminished by cold, while it still uniformly retains its elastic fluidity, but also discover in it properties and behaviour different from those of common air, then I consider myself justified in believing that this is a peculiar kind of air. I say that air thus collected must retain its elasticity even in the greatest cold, because otherwise an innumerable multitude of varieties of air would have to be assumed, since it is very probable that all substances can be converted by excessive heat into a vapour resembling air.”[12]