§ 1C9. A LOINS. 177 



gradually obtained by diffusion, but it is often long before they 

 dejDosit crystals (most easily obtained from Natal aloes). Ferric 

 chloride colours them, without exception, gi-eenish-black (Natal 

 aloin very slowly) ; they are all gradually precipitated by basic 

 acetate of lead ; perchloride of platinum colours Barbadoes and 

 Curasao aloin by degrees red to violet, Socotra and Cape aloin 

 greenish-brown, Natal aloin yellowish-brown ; chloride of gold pro- 

 duces a more or less fine rasplierry-red, passing generally into violet; 

 with strong hydrochloric acid. Natal aloin alone becomes violet; 

 mercurous nitrate colours Barbadoes and Curasao aloin reddish. 

 All the aloins are precipitated from aqueous solution by bromine- 

 v/ater, in the form of sparingly soluble l^rominated compounds, 

 which contain frequently, but not invariably, 40 to 44 per cent, of 

 bromine. The opinion expressed in my ' Chemische Werthbestim- 

 mung starkwirkender Droguen,' that these bromine-precipitates 

 might be used in determining the value of the different varieties of 

 aloes, was leased upon some experiments of Kondracki's;^ but since 

 Treumann has shown that one and the same aloin can yield more 

 than one substitution-product, I have been shaken in this opinion. 

 The applicability of another method of estimating the value of an 

 aloes by ascertaining how much tannin is necessary to precipitate 

 and redissolve one of the constituents, has also been rendered 

 doubtful. I Avas convinced that this body precipitable by tannin 

 was a decomposition-product of aloin, or possibly an amorphous 

 modification, and that it acted directly as a purgative; Kondracki's 

 experiments confirmed this supposition by showing that the more 

 active an aloes was, the greater was the amount of tannin solu- 

 tion required in titrating. But as more recent experiments have 

 proved that the aloins themselves when taken in sufficient quantity 

 have a purgative action (whether direct or indirect, I am unable 

 to say), and the attempts to compare the amount of aloin in an 

 aloes with that of the substance precipitated hy tannin have not 

 met with success, I feel myself compelled to retract for the present 

 the statements made in my ' Werthbestimmung ' on this subject. 

 The aloin is accompanied in aloes by a resinous substance which 

 does not dissolve when the aloes is treated with about 10 parts of 

 water, but which is soluble in concentrated a(|lieous aloin-solutions, 

 in hot water, and in alcohol. Another body, probably non-pur- 

 gative, also occurs in dried aloe-juice ; it is freely soluble in cold 

 1 Beitr. z. Kenntniss der Aloe, Diss. Dorpat, 1874. 



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