But Dr. Henderson says, in his learned work, entitled “The History of Ancient and Modern Wines,” p. 342, that the simple test pointed out to him by his friend Dr. Prout is equally satisfactory, and may be applied either to red or white wines. “On adding ammonia to wines, which had the appearance of being genuine, he observed that the precipitate was of an olive green colour; shewing the analogy between the colouring principle and the vegetable blues, most of which are rendered red by acids, and green by alkalis. This conjecture is, in some measure, confirmed by the recent discovery of M. Breton, professor of chemistry in Paris, with respect to the cause of that disorder in wines known by the name of _tournure_. Wine thus affected acquires a disagreeable taste and smell, loses its red colour, and assumes a dark violet hue, which changes are found to proceed from the presence of carbonate of potash, in consequence of the decomposition of the tartar contained in the liquor. To restore the natural colour and flavour, if the disease be not of long standing, it is only necessary to add a small quantity of tartaric acid, which, combining with the potash, forms cream of tartar, as is shown by the subsequent deposition of crystals. Revue Encyclopedique, November, 1823. In genuine wines, the colouring matter seems to partake of the character of a lake, partly held in solution by the excess of acid present, and partly combined with the earthy phosphates; for, in the precipitates obtained from these wines by means of ammonia, it appears in union with the triple phosphate of magnesia. Even the white wines of Xeres, Madeira, and Teneriffe, exhibit this mixed precipitate; their colouring matter being probably derived from the red grapes which enter into their composition. In fictitious wines, on the other hand, such as those procured from the black currant, gooseberry, orange, &c. the last mentioned salt was thrown down by ammonia, but more gradually, in less quantities, and without any admixture.” The method of ascertaining the strength, or quantity of spirit or alcohol in wines is by the following process, for the discovery of which the public is indebted to Mr. Brande.