16. In 1784 he pointed out a method by which _citric acid_ may be obtained in a state of purity from lemon-juice. He likewise determined its characters, and showed that it was entitled to rank as a peculiar acid. It was during the same year that he observed a white earthy matter, which may be obtained by washing rhubarb, in fine powder, with a sufficient quantity of water. This earthy matter he decomposed, and ascertained that it was a neutral salt, composed of oxalic acid, combined with lime. In a subsequent paper he showed, that the same oxalate of lime exists in a great number of roots of various plants. 17. In 1786 he showed that apples contain a peculiar acid, the properties of which he determined, and to which the name of _malic acid_ has been given. In the same paper he examined all the common acid fruits of this country--gooseberries, currants, cherries, bilberries, &c., and determined the peculiar acids which they contain. Some owe their acidity to malic acid, some to citric acid, and some to tartaric acid; and not a few hold two, or even three, of these acids at the same time. The same year he showed that the syderum of Bergman was phosphuret of iron, and the _acidum perlatum_ of Proust _biphosphate of soda_. The only other publication of Scheele, during 1785, was a short notice respecting a new mode of preparing _magnesia alba_. If sulphate of magnesia and common salt, both in solution, be mixed in the requisite proportions, a double decomposition takes place, and there will be formed sulphate of soda and muriate of magnesia. The greatest part of the former salt may be obtained out of the mixed ley by crystallization, and then the magnesia alba may be thrown down, from the muriate of magnesia, by means of an alkaline carbonate. The advantage of this new process is, the procuring of a considerable quantity of sulphate of soda in exchange for common salt, which is a much cheaper substance. 18. The last paper which Scheele published appeared in the Memoirs of the Stockholm Academy, for 1786: in it he gave an account of the characters of gallic acid, and the method of obtaining that acid from nutgalls. Such is an imperfect sketch of the principal discoveries of Scheele. I have left out of view his controversial papers, which have now lost their interest; and a few others of minor importance, that this notice might not be extended beyond its due length. It will be seen that Scheele extended greatly the number of acids; indeed, he more than doubled the number of these bodies known when he began his chemical labours. The following acids were discovered by him; or, at least, it was he that first accurately pointed out their characters: Fluoric acid Molybdic acid Tungstic acid Arsenic acid Lactic acid Gallic acid Tartaric acid Oxalic acid Citric acid Malic acid Saclactic Chlorine.