SUGARS. 53 



way : To the suspected liquid in a test tube, we add one or 

 two drops of a moderately strong solution of sulphate of 

 copper, and render the mixture distinctly alkaline by the 

 addition of caustic potash in solution. On the addition of 

 the alkali the mixture will assume a distinctly blue color, 

 especially marked if sugar be present. On the application 

 of heat, if sugar be present, a little before the liquid reaches 

 the boiling point, a yellowish or reddish precipitate will 

 begin to show itself in the upper part of the test tube, which 

 as the heat continues will gradually extend through the whole 

 of the liquid. If no sugar be present, the liquid will retain 

 its clear blue color, unless the boiling be prolonged, when a 

 black precipitate of the black oxide of copper is likely to 

 appear. In this test, before the heat is applied, the copper 

 is in the form of the sulphate of a protoxide (CuO, SO 3 ), 

 which is soluble ; but on boiling in an alkaline solution, 

 the sugar becomes oxidized, is transformed into an acid, 

 the nature of which is not well determined, and the copper, 

 losing an equivalent of oxygen, becomes reduced to the con- 

 dition of a sub-oxide (Cu 2 O), which has a reddish or yellow- 

 ish color, and is insoluble. This is the way in which the test 

 is generally employed. Trommer recommended (1841), with 

 special reference to examination of urine, to first add the 

 solution of potash, then filter, and then add the solution of 

 copper. If sugar be present, a reduction of the sub-oxide will 

 take place spontaneously in a few hours, or may be produced 

 immediately by boiling. This removes certain sources of 

 obscurity in exarnining the urine, which result from a pre- 

 cipitate produced by the simple action of the potash, and not 

 dependent on the presence of sugar. 



If care be taken to employ the following simple precau- 

 tions in the application of this test, it will be found the most 

 reliable and simple of any that are in use for qualitative 

 analysis. 



The solution to be examined must be clear. A clear 

 extract of the blood, muscles, or liver, is easily made in the 



