266 THE CONSTITUENTS OF BILE. [CH. XI. 



acid settles to the bottom. Gently shake the test-tube from side to 

 side. As the fluids gradually mix a deep purple colour develops. 



NOTES. i. This reaction depends on the production of furfurol from the 

 cane-sugar by the strong acid. (See Ex. 114.) 



2. If too much cane-sugar be taken the fluid will turn brown or black, 

 owing to the charring produced. 



3. Proteins give a very similar reaction with furfurol in the presence of 

 strong acids. (See Ex. 26.) Proteins also tend to give a brown char with 

 sulphuric acid. For these reasons it is advisable to remove the proteins from 

 solution before attempting the test. 



4. The purple colour obtained is only stable in the presence of strong 

 sulphuric acid. It disappears on dilution with water. 



5. If a small portion of the coloured fluid be diluted with 50 per cent, 

 sulphuric acid, and examined with the spectroscope, two absorption bands will 

 be seen, one between the lines C and D, nearer the latter ; the other in the 

 green, overlapping E and B. 



6. The test cannot be applied directly to urine, owing to the presence of 

 chromogenic substances that yield intense colours with sulphuric acid. 



316. Hay's test for bile salts. Take 10 cc. of the solu- 

 tion in a test-tube. Sprinkle the surface with flowers of sulphur and 

 note that they fall through the liquid to the bottom of the tube. 

 Repeat the test with water, noting that the particles remain on the 

 surface. 



NOTES. i. This test for bile salts depends on the remarkable property 

 that they possess of lowering the surface tension of water, thus enabling the 

 particles of sulphur to sink through the fluid. 



2. The test is of great value for the detection of bile salts in urine. 



3. This property of bile salts is utilised by draughtsmen in preparing 

 tracings on oiled paper, on which ink collects in drops, and does not spread 

 well. If the paper be first treated with a little ox gall and allowed to dry the 

 difficulty is removed, owing to the reduction in surface tension. 



4. A method for estimating bile salts in urine has been described by 

 Grunbaum, depending on this property. The rate of escape of the urine from 

 standard capillary tubes is noted, the rate increasing with the concentration of 

 bile salts. 



317. Oliver's test for bile salts. Acidify 5 cc. of the solution 

 with two or three drops of strong acetic acid, filtering if necessary. 

 To the acid solution add an equal quantity of i per cent, solution of 

 peptone. A white milkiness or a decided precipitate is produced, 

 insoluble in excess of acid. 



NOTES. i. The precipitate formed consists of a compound of protein 

 with bile acids. 



2. The test can be applied to urine. (Ex. 379.) 



