90 CLASSICS OF MODERN SCIENCE 

 nitre; and Hoffman, finding it composed of the magnesia united to 

 an acid, obtained a separation of these, either by exposing the com- 

 pound to a strong fire, in which the acid was dissipated, and the 

 magnesia remained behind, or by the addition of an alkali, which 

 attracted the acid to itself : and this last method he recommends as 

 the best. He likewise makes an inquiry into the nature and virtues 

 of the powder thus prepared ; and observes, that it is an absorbent 

 earth, which joins readily with all acids, and must necessarily destroy 

 any acidity it meets in the stomach ; but that its purgative power is 

 uncertain, for sometimes it has not the least effect of that kind. 

 As it is a mere insipid earth, he rationally concludes it to be a pur- 

 gative only when converted into a sort of neutral salt by an acid 

 in the stomach, and that its effect is therefore proportional to the 

 quantity of this acid. 



Although magnesia appears from this history of it, to be a very 

 innocent medicine; yet, having observed that some hypochondriacs, 

 who used it frequently, were subject to flatulencies and spasms, he 

 seems to have suspected it of some noxious quality. The circum- 

 stances, however, which gave rise to his suspicion, may very possibly 

 have proceeded from the imprudence of his patients ; who, trusting 

 too much to magnesia (which is properly a palliative in that disease) 

 and neglecting the assistance of other remedies, allowed their dis- 

 order to increase upon them. It may, indeed, be alleged that magnesia, 

 as a purgative, is not the most eligible medicine for such constitutions, 

 as they agree best with those that strengthen, stimulate, and warm; 

 which the saline purges, commonly used, are not observed to do. But 

 there seems at last to be no objection to its use, when children are 

 troubled with an acid in their stomach: for, gentle purging, in this 

 case, is very proper; and it is often more conveniently procured by 

 means of magnesia, than of any other medicine, on account of its 

 being entirely insipid. 



The above-mentioned Author, observing, some time after, that a 

 bitter saline liquor, similar to that obtained from the brine of salt- 

 petre, was likewise produced by the evaporation of those waters which 

 contain common salt, had the curiosity to try if this would also yield 

 a magnesia. The experiment succeeded : And he thus found out 

 another process for obtaining this powder; and at the same time 



