Biographical Memoir of' the late Dr Henry. 7 



tinguished him in that most delicate province of chemical re- 

 search. 



In the same interval (1800) he also made public, through the 

 Philosophical Transactions, some experiments on the muriatic 

 acid gas. This memoir is one of peculiar interest, both as re- 

 calling the state of chemical docti'ine, anterior to the grand 

 discoveries of Davy, and as marking the influence of pre-con- 

 ceived theories on the interpretation of facts. Oxygen was 

 then regarded as the sole principle of acidity, and the muriatic 

 acid was consequently supposed to be constituted of oxygen, 

 associated with an unknown radical. It was in the hope of 

 detaching this imaginary element from oxygen, that Dr Henry 

 exposed the muriatic acid gas, both alone, and mingled with 

 gaseous matter possessing a strong affinity for oxygen, to re- 

 peated electrical discharges. When the muriatic acid gas was 

 electrified alone over mercury, its volume was uniformly dimi- 

 nished, hydrogen gas was disengaged, and a white deposit was 

 collected, which proved to be calomel. The decrease of volume 

 and the formation of calomel were much more considerable, 

 when the electric discharges were passed through a mixture of 

 oxygen and muriatic acid gas. When the electrization was 

 performed without the presence of mercury in a glass-tube, 

 closed by stoppers, each perforated with platina wire, chlorine 

 was evolved and detected by the usual test. It is manifest 

 that these experiments, had they been justly interpreted, were 

 sufficient to establish the true view of the composition of mu- 

 riatic acid gas. Yet, governed by the theory of acidification 

 then universally prevalent, Dr Henry referred the disengage- 

 ment of hydrogen to the decomposition of water, which was 

 supposed to be still present in the gas after a week's contact 

 with fused chloride of calcium. Nor was it until many years 

 subsequently that the simpler theory was firmly established by 

 the genius of Davy. To the new doctrine, Dr Henry had, 

 however, the merit of becoming an early convert ; and in a 

 supplementary essay, published in the Transactions for 1812, 

 he supplied some impot tant evidence in its favour. He shewed 

 that the same proportion of hydrogen gas was obtained by 

 electrizing muriatic acid gas, whether it had been exposed or 

 not to fused chloride of calcium ; and hence concluded that 



