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 Casiri, that his works were numerous; but only two of them have been published, namely, his Treatise on Optics, and that on the Twilight. They were both published in Latin, in 1572, by Frederic Risner, under the title of Opticæ Thesaurus. ALIMENTS. See in the Encyclopædia, and in this Supplement. ALKALIMETER, a name given to a small instrument, invented by M. Descroizilles, for ascertaining the value of the alkalis in commerce. There is an account of it in the 60th volume of the Annales de Chimie, which is translated in the 28th volume of Tilloch’s Philosophical Magazine. ALLAN, a Scotch historical painter of considerable celebrity, was born at Alloa, on the 13th February 1744. At a very early age he showed such marks of genius as attracted the notice of some gentlemen living in the neighbourhood. In a remote part of the country, and deprived of the ordinary means of indulging his propensity to drawing, he betook himself, when a boy, to such implements and materials as he could readily procure; and the mechanical skill and taste which, in particular, he displayed in using his knife, have been mentioned as remarkable for his years. Mr Stewart, then Collector of the Customs at Alloa, having mentioned these proofs of natural talent to Mr Fowlis the Painter, who sometime before had instituted an Academy in Glasgow, for painting and engraving, invited young Allan to study under his care. Here he remained about seven years, studying the elementary principles of his art; and by the proficiency which he attained, justified the opinion of his talents, which had procured him admission to that ill-fated seminary. But, although, the public taste for the fine arts, which then existed in Scotland, was so feeble ag to leave his preceptor without that support which his liberal and spirited efforts justly claimed, Allan, on leaving the academy, had the good fortune to gain the patronage of individuals, whose generosity enabled him to prosecute his views, and to improve his taste, by studying the works of art abroad. At the joint expence of several persons of fortune, particularly Lord Cathcart and Mr Abercrombie of Tullibody, he was enabled to go to Italy; and at Rome he devoted himself with great zeal to his profession. Here he remained for no less a period than sixteen years, during which time, his subsistence chiefly depended on the copies which he made from the most celebrated pictures of the ancient masters; but one original work which he then painted, does the highest credit to his talents, namely, his composition representing the Origin of Painting. On this picture, indeed, the fame of Mr Allan, as a historical painter, chiefly rests; and its merit was rewarded, by his receiving the gold medal given by the Academy of St Luke, in the year 1773, for the best specimen of historical composition. There is an excellent engraving of this piece by Cunego.

On his return to his native country, he took up his residence in Edinburgh, where he was appointed master of the Academy established by the Trustees for Manufactures in Scotland. There he executed great variety of works, of various degrees of merit; but perhaps none such as might have been expected from the author of the Origin of Painting. Those, indeed, by which he is most known, are of a cast altogether different, being remarkable for the comic humour which they display. The Scotch wedding; the Highland dance; the Repentance-stool, with his Illustrations of the Gentle Shepherd, are all of this class, and so generally known, as to need no description. Four pictures representing the Carnival, are said to have been very successful. Of his graver compositions, the Prodigal Son, in possession of Lord Catheart, and his Hercules and Omphale, in the possession of Mr Erskine of Mar, are regarded as works of great merit.

As an artist, Mr Allan possessed much facility of invention; with a keen discernment of those evanescent circumstances in the outward form, which mark the different shades of passion and affection in the mind. With the talent of artfully arranging the various parts of a crowded and bustling scene,—such as appears in some of his comic productions, he possessed that feeling for simplicity, which gives to his graver compositions, a classical and antique air of elegance. In his drawing he often was hasty and incorrect. He executed a great variety of etchings and drawings in water colour, which are valuable, on account of their excellent humour, and the great knowledge of national character which they express. Many of the original plates and drawings are now in the possession of Mr George Thomson, to whose kindness and friendship their author was much indebted. We do not know that Allan left any pupil who has followed his particular line in painting. Mr H. W. Williams received from him the rudiments of art, but his genius has directed him to a different course, which has conducted him to merited eminence as a landscape painter.

Mr Allan is remembered and spoken of as an excellent private character. He died at Edinburgh on the 6th of August 1796, in the 53d year of his age.(.) ALUM, a salt very much employed by Dyers and other artists, in their different processes. It has a white colour, an astringent and acid taste, and crysstallizes in regular octahedrons. Its specific gravity is 1.731, according to a mean of the experiments of Fahrenheit, Wallerius, Watson, and Hassenfratz. Water at the temperature of 60° dissolves about one fifteenth of its weight of alum; while boiling water dissolves about three-fourths of its weight. When alum is exposed to a dry atmosphere, it effloresces slightly. When heated, it speedily becomes liquid. If the heat be continued, its water is driven off, and it loses about 44 per cent. of its weight. What remains, is a vitreous looking substance, known by the name of burnt alum. When alum is exposed to a strong red heat, it gives off a quantity of oxygen gas, as Dr Milner first ascertained. The same fact was afterwards pointed out by Gay-Lussac.

Alum is one of those substances which chemists have distinguished by the name of triple salts. It is composed of sulphuric acid, potash, alumina and water. That it contained sulphuric acid as a constituent, was known even to the Alchemists. Pott and Margraaf demonstrated, that alumina was another constituent. Mr Pott, in his Lithogeognosia, showed Rh