HYDROCARBONACEOUS SUBSTANCES. 55 



It is also found in the juices of many plants, in various sweet fruits, 

 and in honey, where it is associated with certain other varieties. It is 

 freely soluble in water. Its solution has a moderately sweet taste, and 

 deviates the plane of polarization toward the right 53.5. 



It is this form of sugar which is produced from starch by boiling 

 with dilute acids, by the action of the digestive fluids, and in the plant 

 during germination. The change consists in the assumption by starch 

 of the elements of water, the new substance thus produced being still 

 a carbo-hydrate. The transformation of starch into glucose is there- 

 fore represented as follows : 



STARCH. WATER. GLUCOSE. 

 C 6 H 10 5 + H 2 = C 6 H 12 6 . 



Glucose may be recognized in solution by various tests. First, the 

 action of alkalies at a boiling temperature. If a solution of glucose 

 be heated with a solution of potassium hydrate, the sugar is decom- 

 posed and the liquid assumes, first, a yellowish and then a brown color, 

 which becomes deeper in proportion to the amount of glucose and alkali 

 in the solution. This is not an exclusive test for glucose, as some other 

 organic matters are discolored in a similar way by the strong alkalies ; 

 but it will serve to distinguish it from cane sugar, which does not pos- 

 sess this property. 



Secondly, the test most commonly employed for glucose depends 

 upon its power of reducing the salts of copper in a boiling alkaline 

 solution. This test, which is known as "Trommer's test," is applied 

 in the following manner : A small quantity of copper sulphate in 

 solution is added to the suspected liquid and the mixture rendered 

 alkaline by the addition of potassium hydrate. The solution then 

 takes a blue color. On boiling the mixture, if glucose be present, the 

 copper suboxide is thrown down as an opaque red, yellow, or orange- 

 colored deposit; otherwise no change takes place. In this reaction 

 the sugar, which is oxidized at a high temperature under the influence 

 of the alkali, takes a portion of its oxygen from the copper salt and 

 reduces it to the form of insoluble suboxide. 



Some precautions are necessary in the use of this test. As a general 

 rule, the quantity of copper sulphate added to the liquid under ex- 

 amination, should be only sufficient to give a distinct blue tinge after 

 the addition of the alkali. If the copper salt be used in excess, the 

 sugar in solution may not be sufficient to reduce the whole of it ; and 

 that which remains as a blue sulphate may mask the yellow color of 

 the suboxide thrown down as a deposit. This difficulty may be removed 

 by due care in the proportion of the ingredients. 



Furthermore, there are some albuminous substances which interfere 

 with the test, and prevent the reduction of the copper, even when 

 sugar is present. Certain animal matters, to be described hereafter, 

 which are liable to occur in the gastric juice and in the blood, have 

 this effect. 



The ordinary ingredients of the urine also interfere with Trommer's 



