104 A Tribute to the Memory of [Sept. 



having made himself sufficiently master of what was ascertained 

 in that department of knowledge, he felt an ambition to extend 

 its boundaries. In the year 1771, he communicated to the 

 Royal College of Physicians of London, " An improved Method 

 of preparing Magnesia Alba," which was published in the second 

 volume of their Transactions. Two years afterwards it was 

 reprinted, along with essays on other subjects, in a separate 

 volume, which was dedicated by Mr. Henry to his friend Dr. 

 Percival. 



The calcination of magnesia had, at that time, been practised 

 only in connexion with philosophical inquiries. Dr. Black, in an 

 essay, which is still perhaps not surpassed in chemical philoso- 

 phy, as a beautiful example of inductive reasoning, had fully 

 established the differences between magnesia in its common and 

 in its calcined state ; but he does not appear to have made trial 

 of the pure earth as a medicine, though several inconveniences, 

 from its use in the common form, had long before been pointed 

 out by Hoffman.* On this subject, Mr. Henry's claims extend 

 to the free disclosure of his improvements ; to the early and 

 strenuous recommendation of the medicinal use of pure magne- 

 sia ; and to the discovery of some of its chemical agencies. It 

 is but justice to him to state that his recommendation of its 

 employment as a medicine was perfectly disinterested ; for it 

 was not till his work was printed, and on the eve of issuing from 

 the press, that the preparation of magnesia for sale was suggested 

 to him by a friend, in a letter relating to the intended publica- 

 tion, which is still preserved as a part of his correspondence. 

 Before carrying this suggestion into effect, he thought it proper 

 to consult Sir John Pringle, Sir Clifton Wintringham, Dr. War- 

 ren, and some other leading members of the College of Physi- 

 cians, as to their opinion of the propriety of the measure ; and 

 he did not adopt it unril those gentlemen had declared it to be 

 not more adviseable on his own account than on that of the 

 public. 



Soon after the publication of the small volume of Essays, Mr. 

 Henry found himself involved in a controversy, arising out of 

 some remarks in the appendix, respecting which, as the subject 

 was of temporary interest, it is unnecessary to enter into parti- 

 culars. It is sufficient to state that the accuracy of some of his 

 experiments, which had been called in question, was confirmed 

 by the concurrent testimony of Dr. Percival and Dr. Aikin ; and 

 that the chemical properties, first ascertained by him to belong to 

 pure magnesia, were considered by Bergman and by Macquer as 

 worthy of being incorporated into their respective histories of 

 that earth. 



It was probably in consequence of the publication of these 

 Essays, that Mr. Henry was admitted into the Royal Society of 



* Hoffman. Oper. torn. iv. p. Sisl . 



