§ 1G9. ALOINS. 177 



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

 deposit crjratals (most easily obtained from Natal aloes). Ferric 

 chloride colours them, without exception, greenish-black (Natal 

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

 acetate of lead ; perchloride of .platmiun 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 raspbeiry-red, passing generally into violet; 

 with strong hydrochloric acid, Natal aioin alone becomes violet; 

 mercurous nitrate colours Barbadoes and Cura9ao aloin reddish. 

 All the aloins are precipitated from aqueous solution by btomine- 

 water, in the form of sparingly soluble brominated compounds, 

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

 bromrjie. 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 based upon some experiments of !^ondracki'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 was convinced that this body precipitable by tannin 

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

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

 experiments confirmed this supposicion by showing that the. more 

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

 tion required in titrating. But as more recent e^eriments 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 by tannin have not 

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

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

 The aloin is accompanied in aloes by a resinous substance which 

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

 water, but which is soluble in concentrated aqueous aloin-solutions, 

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

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

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



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