54 SUBSTANCES SOLUBLE IN ALCOHOL. 



examined. If, on the other hand, this substance be soluble in 

 water, its isolation may be attempted by the method of agitation. 



An experiment should also be made to ascertain the action of 

 the glucose thus produced on a ray of polarized light, as well as 

 its behaviour with yeast that is, its capability or incapability of 

 entering into fermentation. For this purpose it is best to decom- 

 pose the glucoside with sulphuric acid, which may be subsequently 

 removed by carbonate of barium. The filtrate from the sulphate 

 of barium, which should be faintly acid in reaction, should be 

 mixed with a little yeast, introduced into a eudiometer over 

 mercury, and observation made whether, under these circum- 

 stances, carbonic-acid gas is evolved. Many glucosides yield, 

 besides glucose, products of decomposition which are antagonistic 

 to alcoholic fermentation ; these are, if possible, to be removed. 



This fermentation experiment will have a particular value in all 

 cases in which the glucoside itself reduces alkaline copper solution. 

 Proof of the glucosidal nature of the substance may then be 

 found in the experiment yielding a negative result before, but a 

 positive one after, the action of the dilute acid, especially if the 

 substance be soluble in ether or cold absolute alcohol. Saccharoses 

 and other carbohydrates, which would yield similar results, are 

 thus excluded, they being insoluble in the liquids named. 



It is hardly necessary for me to point out the desirability of 

 estimating, by means of Fehling's copper solution, the sugar pro- 

 duced by the decomposition of a glucoside. (Cf. 83, et seq. ; 

 200, et seq.) 



Some bodies which are usually treated of with the glucosides 

 yield, when acted upon by acids, not glucose, but substances allied 

 to sugar or mannite, which, like isodulcite, are unfermentable. 

 (Cf. 212.) 



62. Sulphuric Acid Group-reaction. Many glucosides are capable 

 of acting like sugar when mixed with bile and sulphuric acid 

 that is, of producing a red colour. This reaction has been described 

 as to a certain extent characteristic of the whole group of gluco- 

 sides ; but it should be remarked that among them there are many 

 which are reddened by sulphuric acid alone, whilst some cannot 

 replace sugar in the test for bile ; and others, when mixed with 

 sulphuric acid, assume such characteristic colours that the bile 

 reaction is quite undistinguishable. 



For further information concerning glucosides, compare 165, 

 et seq. 



