368 DETECTION OF SUBSTANCES. [CH. XIV. 



Diastatic enzymes are probably absent if the solution is strongly acid 

 or alkaline. The reaction and salt content are factors of importance. The 

 best method of testing for these enzymes is some modification of Ex. 237, 

 adding the buffer and the salt for the reasons given in that exercise. The 

 solution should be carefully neutralised to litmus before making the tests. 



The enzymes acting on the disaccharides are usually more difficult to 

 identify. Sucrase is sometimes found in a very active condition, but the tests 

 for maltase and lactase generally require an incubation period of at least 

 15 hours. For details see Exs. 266-268. 



Lipase is rather unstable to acids. For tests see Ex. 168. 



A few special hints on the examination of physiological fluids. 



1. It is impossible to obtain a heat coagulum of albumin or 

 globulin in an acid or alkaline fluid. The reaction must be neutral 

 or only very faintly acid. 



2. A little litmus solution in the fluid does no harm, and often 

 reminds one that the reaction changes after boiling (owing to the 

 evolution of CO 2 ). 



3. In testing for peptones, after removing the albumoses by 

 saturation with ammonium sulphate, the biuret test succeeds only if 

 at least two volumes of 40 per cent, soda are used. The test will not 

 be obtained with the ordinary 5 per cent, soda (see notes to Ex. 57). 



4. Gelatin reacts very much like the albumoses, except that 

 it does not yield the glyoxylic reaction. It can be precipitated by 

 half-saturation with ammonium' sulphate. If the precipitate is 

 collected, squeezed and dissolved in a very little hot water, the solu- 

 tion will often set after being thoroughly cooled for some time. 



5. It is impossible to obtain Fehling's or Benedict's test for 

 the reducing sugars in the presence of any considerable amount of 

 ammonia or ammonium salts. 



6. The sugars reduce only in an alkaline medium. If the 

 fluid under examination be acid, it must be neutralised before 

 boiling with the Fehling's or Benedict's solution. 



7. In testing for cane sugar do not forget that starch and the 

 dextrins are hydrolysed to glucose by boiling acids. But whereas 



