Strabo relates, that a glass manufacturer of Alexandria informed him that an earth was found in Egypt, without which the valuable coloured glass could not be made. It has been thought by some, the glass earth here meant was a mineral alkali which was readily found in Egypt, serving to make glass; but this author speaking expressly of coloured glass, it has been suggested as probable, the alkali above named could not have reference to what the artisan intended to imply, but that it must be referred to some metallic earth or manganese. One Democritus is named by Seneca, as having discovered an art of making artificial emeralds; but it has been conjectured that what the philosopher meant was the art of communicating colour to natural rock crystal, or colouring glass already made, so as to resemble stones, which is a process performed by cementation. Directions have been furnished for this purpose by Porta, Neri, and others; but it is discovered that the articles so coloured are liable to such accidents in the process, that it is next to impossible to render things of any size tolerably perfect, so as to bear cutting afterwards. In the Museum Victorium at Rome, there are shown a chrysolite and an emerald, both perfectly well executed, and thoroughly transparent, without a blemish. We have not from the ancients an account of what process they employed; but it must be evident that nothing less than metallic calces could have been used; and for this evident reason, that any other substance could not have resisted the influence of the necessary heat. The last century has, however, produced certain artists in northern European nations, who have adopted a method of employing the precious metals, to communicate a tincture to glass in the process of making, where iron, &c. were originally only used; and their endeavours have been attended with singular success. By means of an amalgam of gold, or a solution in _aqua regia_, and precipitated with a solution of tin, the metal then assuming the appearance of a rich purple coloured powder; so prepared, it is mixed with the best _frit_, and then called the precipitate or gold calx of Cassius, the inventor of gold purple, or mineral purple. This precipitate communicates a rich ruby coloured purple, so perfect that it is impossible to discover the deception, without the substances be tried by the usual means--cut with a diamond or a prepared file. We have had in England some very eminent artists in the practice of staining glass, and also for making artificial representations of various precious stones.