hitelligcjice and Miscellaneous Articles. 543 



be a question whether at comparatively low temperatures, 32° of Fahr., 

 for instance, this power is, or is not, completely suspended. This 

 winter, if opportunity offers, a few experiments shall be made to de- 

 termine this point, and their results, if of any interest, noticed. 



J. Dknham Smith, 



METHOD OF DETECTfNG SULPHUROUS ACID IN THE HYDRO- 

 CHLORIC ACID OF COMMERCE. 



This process is founded on the action that protochloride of tin exerts 

 on sulphurous acid. Pelletier, sen., noticed {Ann. de Cliimie, tome 

 xii. page 231, 1792,) that when solutions of these substances are 

 mixed, the salt of tin deoxidizes the sulphurous acid, and affords a 

 beautiful yellow precipitate, consisting of Sulphur and peroxide of tin. 



The mode of proceeding is as follows : Put about half an ounce of 

 the hydrochloric acid which is to be tested into a glass, and then add 

 about a quarter of an ounce of the salt of tin, such as is very white 

 and not altered by exposure to the air : stir these togetiier, and then 

 add about two or three times its bulk of distilled water, and mix well. 



When the hydrochloric acid does not contain sulphurous acid, no 

 partic'dar action ensues on the addition of the chloride of tin and the 

 water ; the salt of tin dissolves, and the solution is merely rendered 

 slightly turbid by the action of the air. 



But should the hydrochloric acid contain sulphurous acid, it will 

 be seen, that on the addition of the salt of tin, the acid becomes tur- 

 bid and yellow, and then, on adding the distilled water, the odour of 

 hydiosulphuric acid will be easily distinguished, the liquid will be- 

 become of a brown tint, and deposit a powder of the same colour. 

 These pliaenomena are so distinct that we cannot hesitate for an in- 

 stant as to the presence or absence of sulphurous acid. 



Sometimes the brown coloui is not developed until after the lapse 

 of a few minutes, and the larger the proportion of sulphurous acid 

 the darker is the shade produced. The disengagement of hydrosul- 

 phuric acid occurs at the time the mixture is diluted with water ; by 

 standing, the liquor deposits a yellowish brown powder, consisting of 

 sulphuret and peroxide of tin. The reason of this curious reaction is 

 easily explained. One portion of the salt of tin is converted into 

 perchloride at the expense of another portion of this compound, and 

 the tin set free from this decomposition acts on the sulphurous acid 

 so as to produce at the same time both peroxide and protosulphuret 

 of tin. The hydrosulphuric acid which is disengaged immediately 

 after the addition of the water is owing to the hydrochloric acid acting 

 on a part of the sulphuret of tin ; chloride of tin is again formed, and 

 hydrosulphuric acid gas liberated. 



To obtain the phaenomena described it is necessary to add the salt 

 of tin to the hydrochloric acid before the addition of the water, for if 

 we commence by diluting the acid, the addition of the salt does not 

 produce any discolouration. This test is one of such delicacy and ac- 

 curacy that M. Girardin, the author, assures us that it will detect a one 

 hundredth part of sulphurous acid in a sample of hydrochloric acid. — 

 Ann. de Chim. ct de Vhyx.y March 1836. 



