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!
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.
I have particularly selected the Peruvian bark, as requiring a very large quantity of water to extract the whole which it is capable of yielding; but the proportion of water which I have here allowed, will not be too great in obtaining extracts from most vegetable substances; and how greatly not only the quantity, but the quality of the medicine must be affected by the admixture of such a weight of insoluble calcareous earth, is so obvious, that it is needless to expatiate on it.
CHAP. III.
On the medicinal properties of MAGNESIA ALBA.
THE medicinal uses to which Magnesia has hitherto been applied are in general so well known, that it will be necessary only to give a short summary of the cases in which it is beneficial, for the information of young practitioners, and of those of my readers who may not be acquainted with medical subjects, this medicine being frequently administered without the advice of a physician. If it should appear in the subsequent part of this treatise that Magnesia is possessed of any properties hitherto unsuspected in it, the sagacious reader will in a great measure be left to draw his own practical inferences therefrom.
Magnesia Alba is a powerful absorbent, and is given with great success in disorders of the stomach and bowels arising from acidity. This preparation had been introduced into the materia medica abroad several years before it attracted the attention of our countrymen. The celebrated Hoffman having strongly recommended it to the medical world, some English practitioners began to prescribe it, and Dr. Cadogan bestowing high encomiums on it, in his treatise on the nursing and management of children, it soon made its way into general practice, and supplied the place of the testaceous powders and chalk, which before this period were the medicines usually given to correct acidities in the primæ viæ. The acquisition of this medicine was of the more importance, on account of its entire and easy solution in acids, and of the purgative quality which it possesses; whereas the common absorbents are apt to form concretions, and to induce costiveness; strong objections to their free exhibition, as these properties render them peculiarly unfit for the bowels of tender infants who are particularly liable to diseases of this class.
This tendency to acidity generally attends children during the first months and the time of dentition, and discovers itself when too redundant by the green stools, sour vomitings, gripes and purgings which it occasions: and as the nerves of children are extremely irritable, spasmodic affections are often the consequence of this acrid stimulus being retained in their bowels. In these cases Magnesia may be administered in doses from five to twenty or thirty grains, according to the age of the infant; and in proportion as it is intended to act, either as an alterative, or as an easy purgative.