68. Fehling's solution. Keep the two solutions in separate
bottles and mix a few mils of each (equal parts) when ready to make
a test. A deep clear blue fluid is the result of the mixture, the
Rochelle salt holding the cupric hydrate in solution. If kept too
long it is apt to decompose. If in doubt as to the efficiency of the
solution boil it, and if it remains blue it is good.
Add some of the Fehling's solution to a portion of the glucose;
boil; a yellowish (cuprous hydrate) or reddish (cuprous oxide)
precipitate results.
69. Add to a portion of the glucose solution some strong potas-
sium hydroxide solution and then a very small amount of the sub-
nitrate of bismuth. Boil; a black precipitate results which some-
times forms a mirror on the walls of the test tube. This is known as
Boettger's test. Albumin because of the sulphur present, gives a
similar reaction and if present must be removed if a reliable sugar
test is to be obtained.
70. The Phenylhydrazine Test. To 5 drops of phenylhydrazine
and 10 drops of glacial acetic acid in a test tube is added 1 mil of a

33



saturated solution of sodium chloride. After siiaking the mixture,
add 3 mills of 5% dextrose solution and boil the contents of the test
tube for about two minutes. The fluid is then allowed to cool slowly
in order that the crystals may form. The canary yellow precipitate
may he examined in from 20 to 60 minutes under the microscope for
the characteristic glucosazone crystals.
