Experiments and Observations on the Following Subjects / 1. On the preparation, calcination, and medicinal uses of Magnesia Alba. 2. On the solvent qualities of calcined Magnesia. 3. On the variety in the solvent powers of quick-lime, when used in different quantities. 4. On various absorbents, as promoting or retarding putrefaction. 5. On the comparative antiseptic powers of vegetable infusions prepared with lime, &c. 6. On the sweetening properties of fixed air.

by Thomas Henry

About a year and half since I transmitted to the College of Physicians an account of a method of preparing Magnesia Alba, equal to that which had been long sold by Mr. Glass of Oxford, and which was generally and deservedly esteemed by others, as well as by myself, to be the standard of purity. Though that gentleman carefully concealed the minutiæ on which the success of his process depended, he had always prepared it with the most laudable attention.

My process was received by the College, and published in the second volume of the Transactions of that truly respectable society, and has since been reprinted in a pamphlet which I lately published, and in which, among other subjects, I have recommended Calcined Magnesia to the attention of the faculty, as a medicine of considerable importance.

Notwithstanding I had been informed, by a relation of Mr. Glass, that he had disposed of his name in the Magnesia business to some persons, for a very valuable consideration, yet I doubted not their adhering to the proper manner of preparing it; but as I have since had occasion to change my opinion, and as the credit of the Calcined Magnesia depends so much on its purity before calcination, I am necessitated to take this method of informing the public of my reasons for declaring that sold under the name of Mr. Glass to be impure, calcareous, and improper for the purpose of calcining.

Two or three years had elapsed since I had seen any of Mr. Glass's Magnesia, except a small quantity which I had preserved, as a standard for the levity of what I prepared myself. But having a mind to calcine some of his, in order to compare it with my own, I sent for a box from Mr. Harrop, an agent of the proprietors in this town. I was surprized, on opening if, to find the Magnesia specifically lighter, to an amazing degree, than any I had formerly seen, insomuch that the six shillings box, which used to contain about four ounces, now only contained an ounce and half, Troy weight: so that this medicine is sold at the rate of two pounds eight shillings the Troy pound, which is not fourteen ounces Avoirdupois. On attempting to dissolve it in the vitriolic acid, I found the solution very imperfect; and on calcining half of the contents of the box, it was with indignation that I discovered this Magnesia, so extolled, so puffed in every newspaper, for its superior purity and goodness, to contain no inconsiderable quantity of calcareous earth; for the pungency of it was very disagreeable in the mouth, and one scruple of it impregnated an ounce of water almost as strongly as so much lime would have done. These are tests, which, tho' much stronger than that of levity which the proprietors have artfully placed as the principal one, they have avoided mentioning, being sensible of its deficient solubility, and that it would not stand the FIERY ORDEAL.

Willing, however, to believe that this impurity might be accidental, though I had reason to think, from the artful conduct above alluded to, that it was not so, I sent for a box of Magnesia, from the agent for the sale of Mr. Glass's Magnesia at Preston. This likewise proved calcareous, though I thought the lime, produced by calcining it, not quite so pungent as the other; it, however, made a strong lime-water. That I might avoid drawing too hasty conclusions, I procured a third box from Chester, which being subjected to the same trials, seemed more impure than either of the other two. This Magnesia formed a very imperfect solution in the vitriolic acid; and the taste of the lime, after calcination, was so very disagreeable, that I was not free from it for some hours. The water impregnated with it was as strong to the taste as common lime-water, and the precipitate which fell from it, on blowing air into it, was as copious as I ever observed from that prepared with stone or oyster-shell lime. The boxes were all purchased from the agents for the sale of Glass's Magnesia, and every box was sealed with his arms, and had every other mark of authenticity. I have retained samples of each in both states.

I have since repeated the above experiments on the contents of two boxes of Glass's Magnesia, the one of which was purchased of Mr. R. Davis, in Sackville-street, Piccadilly, the other of Mr. William Nicoll, in St. Paul's Church-yard. The Magnesia in each proved to be calcareous, and acquired the properties of quick-lime by calcination.

It would be natural for every person, who might wish to give the Calcined Magnesia a fair trial, to obtain Mr. Glass's for that purpose, on the supposition of its being superior to any other; and as the very first taste of it, in that state, would be sufficient to prejudice any one against the farther use of it, I am necessitated, in justice to my own reputation, and to the public, who may otherwise be deprived of a very valuable medicine, to enter this protest against the use of it.

I have fairly and candidly given up to the public what I have found to be the best method of preparing Magnesia, sufficiently pure for every medical and chemical purpose; and I sincerely wish that every apothecary, who has opportunity and leisure, would prepare it himself. But as, from various reasons, there are, I am convinced, too many who omit to do it, and that too little attention is paid to examining into the purity of what is used; and as it also appears the public have been imposed on, where they had reason to think themselves most secure, I have sent to Mr. Johnson, No. 72, St. Paul's Church-yard, and Mr. Ridley in St. James's-street, a quantity of Magnesia, both in a calcined and uncalcined state, which, though not equal in levity, nor quite so costly, even when calcined, as that sold as Mr. Glass's, will, I doubt not, prove to be non-calcareous, and superior to it in every other respect. And if it should appear so to the gentlemen of the faculty, I wish for no preference to those apothecaries who prepare the medicine faithfully; but flatter myself that I have a superior title to the favour of physicians, of my brethren, and of the community, than those persons can claim, who have meanly stooped to secrete a process, the knowledge of which must be beneficial to mankind, and have abused the public confidence.

I shall only add, that so far was I from expecting to find Mr. Glass's Magnesia impure, that I bought it as a standard; that, as I am informed that Gentleman is dead since this inquiry was finished, nothing, less than the reasons I have adduced, could have prevailed on me to have published this Appendix at this season; and that I do not consider him, but the present preparers of the Medicine, as culpable for the adulteration. That I have been favoured with an account of some experiments made by a Physician of considerable eminence, on that Magnesia, the result of which was similar to what I have here recited; and I appeal for proof of the truth of what I have asserted, to every reputable person who may now have any of it in his possession, and will make the experiment; and that having rested my cause on that issue, I mean not to enter into any controversy on the subject.