^ 60, 61. GIUCOSIDES; RECOONITION. 53 



not act well if the lead compound has been previously dried. It 

 is a well-known fact that, in the course of this operation, the 

 bitter principle may be mechanically retained by the sulphide 

 of lead formed. To avoid loss in this way the sulphide may be 

 filtered ofif, washed, dried, powdered, and boiled with alcohol. 

 In evaporating the alcoholic extract care should be taken not to 

 confound crystals of sulphur with the bitter principle, etc. It 

 will often be found advantageous to decompose the lead precipi- 

 tate in alcohol instead of water. The surface-attraction of the 

 sulphide of lead will, nevertheless, be frequently .found useful 

 in retaining foreign bodies, such as colouring matter and the 

 like, whilst bittor principles, etc., pass into solution. 



The directions given for the decomposition of the lead precipi- 

 tates may also be followed in isolating tannic and vegetable acids 

 from such compounds. (See also § 162.) 



§ 61. Glucosides; jRecognUi(m.^-ln proving the glucosidal nature 

 of a substance advantage may be taken of the influence exercised 

 by ferments (saliva, emulsin, myrosin), etc., or dilute acids (accom- 

 pajiied by heat) on glucosides, which, under such circumstances, 

 split up and yield sugar as one of the products of decomposition. 

 It is advisable to ascertain whether the substance itself, in as pure 

 a state as possible, reduces an alkaline solution of copper, either 

 at the ordinary temperature or on boiling. If no reduction takes 

 place the further examination for glucose is much facilitated. It is 

 customary to boil the substance under examination with water 

 containing 1 to 2 per cent, of sulphuric or hydrochloric acid, and 

 test the liquid for sugar from time to time. The rapidity with 

 which decomposition may be thus effected varies very greatly. 

 Some glucosides yield a sugar reaction after boiling for a few 

 minutes oidy) others require several hours. In some cases it is 

 preferable to allow the acid to act under pressure, or in alcoholic 

 instead of aqueous solution. (Cf. §§ 153, 160.) 



The decomposition-products which are formed, together with 

 glucose, from glucosides, are not unfrequently insoluble in water, 

 and therefore render .the liquid turbid in proportion as the re- 

 action proceeds. This peculiarity may be often made use of as 

 proof that decomposition has commencedj especially, in those 

 cases in which the glucoside itself reduces Fehling's copper solu- 

 tion. After the completion of the reaction and the cooling of the 

 liquids the decomposition-product may be filtered off and further 



