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214 THE CARBOHYDRATES.
large colorless six-sided double pyramids or rhomboids with 13.52 per
cent NaCl.
Glucose in neutral or very faintly acid (organic acid) solution under-
goes alcoholic fermentation with beer-yeast: CeHi206=:
2C2H5.0H+2C02.
In the presence of acid milk or cheese the glucose undergoes lactic-acid
fermentation, especially in the presence of a base such as ZnO or CaC03.
The lactic acid may then further undergo butyric-acid fermentation:
2C3H603 = C4H802+2C02 +4H.
Glucose reduces several metallic oxides, such as copper, bismuth,
and mercuric oxide, in alkaline solutions, and the most important
reactions for sugar are based on this fact. 1
Trommer’s test is based on the property that glucose possesses of
reducing cupric hydroxide in alkaline solution into cuprous oxide. Treat
the glucose solution with about \-\ vol. caustic soda and then carefully
add a dilute copper-sulphate solution. The cupric hydroxide is thereby
dissolved, forming a beautiful blue solution, and the addition of copper
sulphate is continued until a very small amount of hydroxide remains
undissolved in the liquid. This is now warmed, and a yellow hydrated
suboxide or red suboxide separates even below the boiling temperature.
If too little copper salt has been added, the test will be yellowish-brown
in color, as in Moore’s test; but if an excess of copper salt has been added,
the excess of hydroxide is converted on boiling into a dark-brown hydrate
which interferes with the test. To prevent these difficulties the so-
called Fehling’s solution may be employed. This solution is obtained
by mixing just before use equal volumes of an alkaline solution of Rochelle
salt and a copper-sulphate’ solution (173 grams Rochelle salt and about
50-60 grams NaOH per liter and 34.65 grams crystalline copper sulphate
per liter). This solution is not reduced or noticeably changed by boiling.
The tartrate holds the excess of cupric hydroxide in solution, and an excess
of the reagent does not interfere in the performance of the test. In
the presence of sugar this solution is reduced.
According to Benedict 2
this test is more delicate if sodium carbonate is
used instead of sodium hydroxide in the preparation of Fehling’s solution.
B6ttger-Alm£n’s test is based on the property glucose possesses
of reducing bismuth oxide in alkaline solution. The reagent best adapted
for this purpose is obtained, according to Nylander’s 3
modification of
Almex’s original test, by dissolving 4 grams of Rochelle salt in 100 parts
of 10 per cent caustic-soda solution and adding 2 grams of bismuth
subnitrate and digesting on the water-bath until as much of the bismuth
1
In regard to the products produced see Neff, Annal. d. Chem. u. Pharm., 357.
2
Journ. of biol. Chem., 3.
3
Zeitschr. f. physiol. Chem., 8.
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