442 Dr. Thomson on the Action of Sulphuretted Acid Gas [Dec. 



Aiken's Dictionary of Chemistry and Mineralogy, published in 

 1807, I find exactly the same experiments related as those pre- 

 viously given by Kirwan in his paper " On Hepatic Air." I 

 conceive, therefore, that these gentlemen did not make any 

 experiments on the subject themselves ; but simply adopted Mr. 

 Kirwan's results ; though the want of a reference to that gen- 

 tleman might at first lead to the notion that the experiments 

 related were their own. Thenard, in his " Traite de Chimie," 

 vol. i. p. 539, informs us, that the action of sulphurous »acid gas 

 on sulphuretted hydrogen gas has been fully examined : that the 

 two gases decompose each other reciprocally, and form water 

 and sulphur ; that the action is instantaneous, if the gases be 

 moist ; but very slow if they be dry ; and that rather more than 

 two parts of sulphuretted hydrogen are requisite to decompose 

 one part of sulphurous acid. 



These were all the circumstances respecting the action of these 

 two gases on each other which I could find in chemical books at 

 the time that I was employed in preparing the fifth edition of 

 my System of Chemistry for the press. As they did not appear 

 at all satisfactory, I found myself under the necessity of omitting 

 all attempts to explain this action in my System, and to place 

 the fact among the list of subjects which required further inves- 

 tigation ; of which I found myself under the necessity of drawing 

 up a pretty copious list. These topics I propose to investigate 

 in succession ; and I shall lay the result of my experiments occa- 

 sionally before the readers of the Annals of Philosophy. 



The gases employed by Kirwan were probably not absolutely 

 pure. He did not examine with sufficient care the volumes of 

 the two gases requisite to produce the greatest condensation ; 

 nor is his account of the properties of the residual gas sufficient 

 to enable us to understand its nature. There is the same want of 

 precision in the account given by Thenard. According to him, 

 rather more than two volumes of sulphuretted hydrogen are 

 decomposed by one volume of sulphurous acid gas, and the 

 result is water and sulphur. In these two gases the weight of 

 the atom is just double the specific gravity (supposing the speci- 

 fic gravity of oxygen gas to be unity). We may, therefore, sub- 

 stitute atom for volume. 



Sulphur Atoms. 



2 atoms sulphuretted hydrogen contain . . 2 atoms + 2 hydrogen 

 2 atom sulphurous acid contains 1 -f- 2 oxygen 



From this it is obvious, that if the two gases decompose each 

 other, and form water and sulphur, they will be completely con- 

 densed when we mix two volumes of sulphuretted hydrogen 

 with one volume of sulphurous acid — proportions which do not 

 tally completely with the statement of Thenard. 



On repeating the experiment over mercury with gases perfectly 

 pure and sufficiently dry, I found that the two gases are com- 



