GLUCOSE. 285 



should be present in more than sufficient quantity to decompose 

 the sulphate of copper ; the latter, which must be added gradually, 

 and in a diluted state, usually gives rise to a precipitate, which dis- 

 appears when the fluid is stirred ; as the quantity of the oxide of 

 copper which is soluble is proportional to the quantity of sugar which 

 is present, very little sulphate of copper must be added at a time, 

 if we suspect that only a little sugar is present in the fluid. On 

 allowing the azure solution thus obtained to stand for some 

 time, there is usually formed a more pure red or yellow powder 

 than the precipitate which is at once thrown down on boiling the 

 fluid. Moreover, very prolonged heating is improper, for there are 

 several substances which by prolonged boiling separate suboxide of 

 copper from alkaline solutions of oxide of copper ; amongst them we 

 may especially name the albuminous substances, which with oxide 

 of copper and potash yield very beautiful azure-blue, or somewhat 

 violet solutions, and by very prolonged boiling, separate a little 

 suboxide of copper, although without the aid of heat they have 

 not this property. 



If a specimen of urine contain very little sugar, or if we are 

 searching for sugar in some other fluid, it is advisable to extract the 

 solid residue with alcohol, to dissolve the alcoholic extract in water, 

 and to apply the potash and sulphate of copper to this solution. 

 By preceding in this manner we usually obtain the reaction in its 

 most distinct manner. If, however, we are seeking for very small 

 quantities of sugar, as for instance in chyle, blood, or in the egg, we 

 must neutralise the aqueous fluid, previously to its evaporation, 

 with dilute acetic acid, in consequence of the solubility of albu- 

 minate of soda or of casein in alcohol, thus preventing any albu- 

 minous body from remaining in solution. If the reaction do not 

 properly manifest itself in the alcoholic extract thus obtained, or if 

 w r e would carry the investigation further, we must precipitate the 

 sugar from the alcoholic solution by an alcoholic solution of potash, 

 dissolve the compound of sugar and potash in water, and now apply 

 the sulphate of copper ; if only a trace of sugar be present, we ob- 

 tain a most distinct and beautiful reaction. 



The fermentation-test has been much extolled as a means of 

 discovering sugar; but independently of the circumstance that the 

 process is very tedious, it yields, to an inexperienced experiment- 

 alist and observer, far less certain results than Trommer's test. 

 On adding yeast to a fluid, the phenomena of fermentation are 

 simply dependent on the development of bubbles of carbonic acid; 

 if this development of gas from a fluid, as, for instance, from dia- 



