4&a DIGESTION AND ABSORPTION 



afterwards separated, if that is desired, by redissolving the precipitate 

 in water and throwing down the proteoses by saturation with am- 

 monium sulphate. The alcoholic nitrate 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 ob- 

 tained. Tyrosin crystallizes characteristically from animal liquids in 

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

 distinct fatty-looking balls, often marked with radial striae and coloured 

 with pigment (Figs. 186 and 187, p. 488). 



Tests for Tyrosin by M timer's Test. Put a small quantity of tyrosin 

 into a test-tube. Add about 3 c.c. of the reagent,* and heat gradually 

 and gently to the boiling-point. A green colour is obtained. 



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



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

 nucleo-albumin) 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) Put a little of the filtrate from (&) or of the original bile into a 

 porcelain capsule, add a drop or two of a dilute solution of cane-sugar, 

 and mix with the bile. Then add a few drops of strong sulphuric acid, 

 and stir; then a few drops more of the sulphuric acid, stirring all the 

 time. A purple colour appearing at once, or after gentle heating, 

 shows the presence of bile-acids (Pettenkofer's reaction). The bile 

 may be diluted before the addition of the sulphuric acid. In this case 

 a greater amount of the acid must be added. Examine the purple 

 liquid in a test-tube with a spectroscope (p. 74). Dilute the liquid with 

 water, adding some sulphuric acid to partially clear up the precipitate 

 caused by the water. Two absorption bands are seen, one to the red 

 side of D, and the other, a stronger and broader band, over and to the 

 right of E. When only a very small amount of bile-salts is present, 

 the reaction is made more sensitive if a solution of f urf uraldehyde (i to 

 i.ooo) is used instead of cane-sugar. 



(d) Hay's Sulphur Test. Sprinkle a little sulphur (in the form of 

 the fine powder known as flowers of sulphur) on the surface of some 

 bile in a small beaker or deep watch-glass. The sulphur will soon sink 

 to the bottom. Repeat with water; the sulphur will float. The 

 reaction is due to the diminution of the surface tension produced by 

 the bile -acids, and succeeds also in a solution of bile -salts. The test 

 is very sensitive. But in stomach contents, vomit, or stools, it rarely 

 gives good results, since alcohol or acetic acid is often present in the 

 gastric liquid, and phenol and its derivatives in intestinal contents, 

 and all of these cause such an alteration in the surface tension that the 

 sulphur sinks. Ether, chloroform, turpentine, benzine and its deriva- 

 tives, anilin and soaps, also vitiate the test in the same way. 



(e) 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 '^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. Where the bile- 

 pigment is present only in traces, some of the liquid may be filtered 



* The reagent for this test is prepared by mixing thoroughly i volume of 

 formalin, 45 volumes of distilled water, and 55 volumes of concentrated 

 sulphuric acid. 



