Intelligence and Miscellaneous Articles. 157 



porcelain capsule ; the mean of two experiments left 182 parts of 

 white powder ; and the atomic weight of sulphuret of tin being 

 1137*63 and that of oxide of tin 935"29, the sulphuret in question 

 left a quantity of oxide nearly corresponding to that of the sulphuret. 

 The result of the reaction of chloride of tin on sulphurous acid, 

 consists then in the precipitation of sulphuret of tin on one hand, and 

 the production of chloride of tin on the other : 2 at. of sulphurous acid 

 and 3 at. of chloride of tin are decomposed into 2 at. of oxide of tin 

 and 6 at. of free chlorine : these last combine with 3 at. of chloride 

 of tin to form the same number of atoms of chloride, and the two 

 atoms of oxide of tin form chloride of tin with the free hydrochloric 

 acid. — Journal de Pharm. Mars 1839. 



SPONTANEOUS DECOLORATIO.V OF TINCTURE OF LITMUS. 

 BY M. VOGEL. 



It frequently happens that the tincture of litmus prepared with 

 boiling water loses its blue colour entirely after a time, and becomes 

 of a bright brown, or wine yellow colour. This alteration of colour 

 has already been observed by M. Desfosses (Journal de Pharmacia, 

 xiv. p. 487) and by M. Chevreul, and undoubtedly also by other 

 chemists. 



The decoloration occurs especially when the tincture has been left 

 for several months undisturbed, and well stopped in bottles which 

 are not completely full : with alcohol the tincture decolorates more 

 slowly than without it, and the decoloration is especially favoured 

 where a quantity of several pounds is ke])t in a bottle. 



The tincture thus becomes yellow, is not spoiled by the change, nor 

 is it unfit for use, for its original colour may be made to reappear in 

 several modes ; first by exposing it to the air, or by agitating it in 

 a bottle with air. Its colour rea23pears also when heated to 122° Fahr. 

 in a receiver over mercury, provided some air be present in the receiver. 



Although it appears probable that the tincture which has been 

 spontaneously decelerated, becomes again blue by the oxidation of 

 the air, (for it forms at first a blue ring on the surface of the liquid,) 

 it requires however so small a quantity of oxygen, that I could 

 scarcely perceive any diminution in the volume of the air, whilst it 

 regained its colour. 



As the litmus of commerce contains a trace of animal matter, I 

 presumed at first that the decoloration was excited by the decom- 

 position of this animal substance, and that carbonate of ammonia 

 was formed ; but experiment did not confirm this suspicion, for on 

 heating the tincture, which had become spontaneously yellow, in a 

 mattrass furnished with a bent tube, neither ammonia nor carbonic 

 acid was evolved, although the liquid became again blue by the 

 increase of temperature. 



As the litmus of commerce almost always contains sulphate of 

 potash also, it appeared to me possible, and even probable, that if this 

 salt were decomposed, the decoloration might be the result of it. I 

 ascertained the presence of sulphate of potash in the litmus on 

 which my experiments were made, in the following manner : I 



