Canton had, as early as 1747, turned his attention, with complete success, to the production of powerful artificial magnets, principally in consequence of the expense of procuring those made by Dr. Gowan Knight, who kept his process secret. Canton for several years abstained from communicating his method even to his most intimate friends, lest it might be injurious to Dr. Knight, who procured considerable pecuniary advantages by touching needles for the mariner’s compass. At length Dr. Knight’s method of making artificial magnets was communicated to the world by Mr. Wilson, in a paper published in the 69th volume of the _Philosophical Transactions_. He provided himself with a large quantity of clean iron-filings, which he put into a capacious tub about half full of clear water; he then agitated the tub to and fro for several hours, until the filings were reduced by attrition to an almost impalpable powder. This powder was then dried, and formed into paste by admixture with linseed-oil. The paste was then moulded into convenient shapes, which were exposed to a moderate heat until they had attained a sufficient degree of hardness. After allowing them to remain for some time in this state, Dr. Knight gave them their magnetic virtue in any direction he pleased, by placing them between the extreme ends of his large magazine of artificial magnets for a second or more, as he saw occasion. By this method the virtue they acquired was such, that when any one of these pieces was held between two of his best ten-guinea bars, with its poles purposely inverted, it immediately of itself turned about to recover its natural direction, which the force of those very powerful bars was not sufficient to counteract. Dr. Knight’s powerful battery of magnets above mentioned is in the possession of the Royal Society, having been presented by Dr. John Fothergill in 1776. POWER OF THE SUN’S RAYS IN INCREASING THE STRENGTH OF MAGNETS. Professor Barlocci found that an armed natural loadstone, which would carry 1½ Roman pounds, had its power nearly _doubled_ by twenty-four hours’ exposure to the strong light of the sun. M. Zantedeschi found that an artificial horse-shoe loadstone, which carried 13½ oz., carried 3½ more by three days’ exposure, and at last arrived to 31 oz. by continuing it in the sun’s light. He found that while the strength increased in oxidated magnets, it diminished in those which were not oxidated, the diminution becoming insensible when the loadstone was highly polished. He now concentrated the solar rays upon the loadstone by means of a lens; and he found that, both in oxidated and polished magnets, they _acquire_ strength when their _north_ pole is exposed to the sun’s rays, and _lose_ strength when the _south_ pole is exposed.--_Sir David Brewster._ COLOUR OF A BODY AND ITS MAGNETIC PROPERTIES.