PRACTICAL EXERCISES 381 



Neutralize the filtrate with dilute acid ; a precipitate will consist of 

 alkali-albumin. If such a precipitate is obtained, filter it off and test 

 the filtrate for proteoses and peptones as in 4 (d), p. 378. Some 

 digestion, and perhaps a considerable amount, may also have taken 

 place in F and F'; less, or none at all in C and C'; and none in the 

 other test-tubes (pp. 306, 354). 



(c) Add a few drops of the glycerine extract to a test-tube con- 

 taining starch mucilage, which has been previously found free from 

 reducing sugar. Put in a bath at 40 C. After a short time abund- 

 ance of reducing sugar will be found, owing to the action of the 

 ferment, amylopsin. 



(d) Leucin a?id Tyrosin. If pancreatic digestion of proteids (fibrin, 

 e.g.) be allowed to go on for some time, part of the peptone first 

 formed may be broken up into leucin and tyrosin. If the ' digest ' 

 be neutralized to separate alkali-albumin, then filtered, and the filtrate 

 concentrated and allowed to stand, a crop of tyrosin crystals will 

 separate out, since tyrosin is only slightly soluble in watery solutions 

 of neutral salts. These crystals having been filtered off, the proteoses 

 (albumoses) and peptones can be precipitated together by alcohol, 

 and afterwards separated, if that is desired, by redissolving the pre- 

 cipitate in water and throwing down the proteoses by saturation with 

 ammonium sulphate. The alcoholic filtrate will contain any leucin 

 that may be present, since that body is moderately soluble in alcohol, 

 as well as traces of tyrosin, which, however, is much less soluble in 

 this medium. On concentration, crystals of both substances will be 

 obtained. Tyrosin crystallizes characteristically from animal liquids 

 in beautiful silky needles united into sheaves, leucin in the form of 

 indistinct fatty-looking balls, often marked with radial striae and 

 coloured with pigment (Figs. 122 and 123, p. 396). 



7. Bile. (a) Test the reaction of ox bile. It is alkaline to litmus. 



(ft) Add dilute acetic acid. A precipitate of bile-mucin (really 

 nucleo-alburnin) falls down. Some of the bile-pigment is also pre- 

 cipitated. Filter. (Pig's bile contains more of the mucin-like sub- 

 stance than ox bile.) 



(c) Dilute the filtrate from (b). Put a little of it into a porcelain 

 capsule, add a few drops of strong sulphuric acid, and a drop or 

 two of a dilute solution of cane-sugar. A purple colour appearing 

 at once, or after gentle heating, shows the presence of bile-acids 

 (Pettenkofer's reaction). Examine the purple liquid in a test-tube 

 with a spectroscope (p. 62). Dilute the liquid if necessary, adding 

 some sulphuric acid to clear up any precipitate caused by the water. 

 Two absorption bands are seen, one to the red side of D, and the 

 other over E. 



(d) Add yellow nitric acid (containing nitrous acid) to a little bile 

 on a white porcelain-slab. A play of colours, beginning with green 

 and running through blue to yellow and yellowish-brown, indicates 

 the presence of bile-pigment (Gmelin's reaction.) The reaction may 

 also be obtained by putting some yellow nitric acid into a test-tube, 

 and then running a little bile from a pipette on to the surface of 

 the acid. The play of colours is seen at the surface of contact. 



