6i HYDROCARBONACEOUS PROXIMATE PRINCIPLES. 



All sources of error of this kind, due to the presence of extraneous 

 substances, may be avoided in delicate examinations, by treating the 

 suspected fluid with animal charcoal, or by evaporating it to dryness, 

 extracting the dry residue with alcohol, and then dissolving the dried 

 alcoholic extract in water, before the application of the test. Either of 

 these processes will remove the substances which interfere with the 

 action of Trommer's test, and will leave the glucose by itself in the 

 watery solution. 



A more delicate form of the copper test for glucose is in the employ- 

 ment of " Fehling's liquor," which is an alkaline solution of a double 

 copper and potassium tartrate. It is made as follows : 



Take Pure crystallized copper sulphate 40 grammes. 



Neutral potassium tartrate 160 " 



A solution of sodium hydrate of the specific gravity 1.12 . 650 " 



The neutral potassium tartrate, dissolved in a little water, is first 

 mixed with the solution of sodium hydrate. Then the copper sulphate, 

 dissolved in 160 cubic centimetres of water, is gradually added to the 

 alkaline liquor, which assumes a clear, deep blue color. The whole is 

 finally diluted with water to the volume of 1154.4 cubic centimetres. 

 If one drop of this liquid be added to one cubic centimetre of a saccha- 

 rine solution and heat applied, it will detect one-fifteenth of a milli- 

 gramme of glucose by the reduction o the copper oxide. One advan- 

 tage of Fehling's liquor as a test is that the quantity of copper salt 

 contained in a given volume is accurately known, and consequently not 

 only the presence but alsd the amount of glucose in any solution may 

 be determined by the quantity of test liquid which it decomposes at a 

 boiling temperature. One cubic centimetre of Fehling's liquor is 

 exactly decolorized by % J^th of a gramme of glucose. 



One inconvenience connected with this test is that Fehling's liquor by 

 exposure to the air and light undergoes an alteration, in which some of 

 its tartaric acid disappears and is replaced by carbonic acid. In this 

 condition it will partially precipitate on boiling, even without the pres- 

 ence of sugar. To guard against this, it should be kept in bottles which 

 are quite full and protected from the light ; and, in every case where a 

 suspected fluid is to be examined for sugar, a small portion' of the test- 

 liquor should be previously boiled by itself, in order to be sure that it 

 has not undergone spontaneous decomposition. Although by exposure 

 to the air and light at a summer temperature, Fehling's liquor may become 

 altered at the end of a week, yet if protected from the light, in carefully 

 closed and full bottles, it can be kept unchanged for two or three years. 



Thirdly, the most marked and distinctive property of glucose, in a 

 physiological sense, is its capacity for fermentation. If a watery solu- 

 tion of pure glucose be left to itself, even exposed to the air, no remark- 

 able change takes place in it. But if a small quantity of beer-yeast be 

 added and the mixture kept at a temperature of about 25 (77 F.), after 

 a short time it becomes turbid. It then develops an abundance of 



