CHAPTER XIV. 



DETECTION OF SUBSTANCES OF PHYSIOLOGICAL 



INTEREST. 



If no indication as to the origin of the substance is 

 available the scope of the analysis is very considerable. 

 The following account is not intended to be exhaustive, 

 but merely to suggest a few methods of attack. Success 

 demands a sound knowledge of the properties and reactions 

 of a large number of substances. Experience, practice and 

 enterprise count for a good deal. Many substances are 

 not detected by student analysts mainly because they 

 forget to test for them. The hints on page 368 should be 

 carefully studied. The student is urged to perform his 

 tests on the smallest amount of material that is likely to 

 give a conclusive result. With a limited supply of the 

 substance for analysis a much greater variety of tests can 

 thus be applied. 



A. Fluids. 



1. A portion may be neutralised and evaporated to dryness on 

 the water bath. This allows for a subsequent extraction with strong 

 alcohol, which serves for the separation of many substances. It 

 should not be started until there is some indication that it may be 

 necessary, as for the separation of sugars from proteins and poly- 

 saccharides, etc. The evaporation must be conducted in neutral 

 solution to obviate any chemical changes produced by hot acids or 

 alkalies. 



2. Note any characteristic smell of urine, bile, etc. In such 

 cases apply tests for characteristic constituents. 



3. Note the colour and appearance of the fluid : opalescence 

 suggests starch, glycogen, or certain protein solutions; coloured 

 fluids suggest bile, blood or urine. 



