To the first assertion I shall only reply, that every person at all conversant in chemistry knows that Magnesia earth is the same, from whatever substance it can be separated in a pure state; that the factitious Epsom salt yields it in as great a degree of purity as the salt of the Epsom water, and that Dr. Alston assures us, the artificial salt "by various and repeated experiments, made in France as well as in Britain, is demonstrated to be every way as good as, yea to be the very same with, the genuine made of the Epsom waters."[g] [g] Alston's Materia Medica, Vol. I. p. 164. As to the other declaration, it will be sufficient to observe that one gallon of Epsom water contains only seven drachms of salt in a dry season, and hardly six drachms in a wet one;[h] and that for this salt to precipitate its Magnesia properly, it is necessary it should be diluted with little more than its own weight of water.[i] Six drachms of salt will yield two drachms of Magnesia. So that to procure a pound of this powder Mr. Ingram must evaporate above sixty gallons of the water, to between five and six pints, before he begins the precipitation. Sure never did empiricism appear so thinly disguised! [h] Ibid. [i] Essays Physical and Literary, p. 163. In the preceding chapter, the necessity of using water free from any calcareous impregnation has been particularly insisted on, and I have, on another occasion,[j] observed that great attention should be given to the purity of the water used in the making of all the saline preparations; and I may add in almost all the operations of pharmacy. Dr. Percival, in his ingenious experiments on water, found a quart of the Manchester pump water to contain upwards of sixty grains of adventitious matter.[k] Suppose therefore, for instance, that in making the extract from a pound of peruvian bark, it be boiled only six times in the quantity of water directed by the London Dispensatory, nine gallons will be consumed in the process; which is a very moderate allowance, six coctions not being sufficient to extract all the virtues of that drug. Dr. Percival boiled half an ounce of bark twenty five times, in so many different pints of water, the last of which had some impregnation, and the residuum gave a deep colour, and considerable bitterness to rectified spirit of wine. If we likewise suppose only one half of the foreign contents of such water to be left by evaporation, then the quantity of calcareous and saline matter, undesignedly mixed with the extract, will be two ounces and two drachms, or nearly equal to the quantity of extract procured from a pound of bark by pure water. Thus this important medicine becomes grossly adulterated, without any such intention in the operator; and I know it is the common practice to use pump water in making it. [j] Vide Percival's Essays, 2d. Edit. p. 321. [k] Ibid. p. 87.