1821.] An Essay on Chemical Analysis. 143 



solution, when, if arsenious acid be present, it will occasion the 

 ^pearance of a fine lemon-yellow colour through the liquid, but 

 will have no action on phosphate of silver. That substance, 

 however, may prevent the yellow colour from appearing, although 

 arsenious acid be actually contained in the solution; but the 

 addition of a very few drops of very dilute pure nitric acid will 

 immediately produce it. If, therefore, both the silver test and 

 the sulphuretted hydrogen concur in indicating the presence of 

 the poison, no reasonable doubt can be entertained respecting 

 it ; but it is certainly an additional satisfaction to reduce a por- 

 tion to the metallic state, or at least to sublime the oxide so aa 

 to render its pecuhar albaceous odour distinctly evident, where 

 enough can be procured for the purpose, though that cannot 

 often be expected." 



. I shall offer one or two observations upon this statement, and 

 I trust Mr. Children will believe that I do not object upon what 

 appears to me frivolous grounds. In the first place, i think it 

 more convenient to employ a solution of sulphuretted hydrogen 

 in water than to pass the gas through the suspected solution ; 

 the same effects are produced, and to pass gas through so small 

 a quantity of fluid as it is sometimes necessary to operate upon, 

 increases the difficulty, but not the certainty of the process. It 

 should be observed that no hydros ulphuret in solution should be 

 employed instead of the mere gas ; and the arsenious acid should 

 be dissolved simply in water without the aid of any alkali ; for 

 this, if I remember rightly,, interferes with the action of the sul- 

 phuretted hydrogen. 



With respect to the employment of a nitric acid, in case phos- 

 phoric salts be present, I entertain considerable doubt. Nitric 

 acid decomposes sulphuretted hydrogen, precipitating the sul- 

 phur, which may interfere with the process if the acid be em- 

 ployed in excess. 



Mr. Children states that he finds that sulphuretted hydrogen 

 gives a decided yellow colour to an ounce measure of distilled 

 water, containing one drop of a saturated solution of arsenious 

 acid, equal to -j-fo"^^ ^^ ^ grain of the solid acid, or about -^_th 

 of the whole weight of the solution. Two or three drops of 

 phosphate of soda prevented its action, but a little very dilute 

 acetic acid immediately produced the yellow colour. Mr. Chil- 

 dren properly observes that the acetic acid ought to be pure, 

 and not such as has been distilled through a metallic worm. 



It appears from this statement that the power of sulphuretted 

 hydrogen as a test of arsenic is very great. It is singular that 

 phosphate of soda should prevent its action ; and I would sub- 

 mit, whether this very circumstance cannot be taken advantage 

 of as an additional proof of the presence of arsenic. 



In a work of this nature, embracing so many and such varied 

 points of the science, it is not to be wondered at that some few 

 inaccuracies should appear. As an example of these, I woulp 



