cooled with ice, in order that chlorates will not be formed.
After all the chlorine has been passed in, it is necessary to be
certain that the mixture is slightly alkaline, since any excess
of free chlorine in the solution prevents the formation of hydrazine.

In a 14-inch evaporating dish are placed 1500 cc.
of c. p. ammonia water (sp. gr. 0.90), 900 cc. of distilled water,
375 cc. of 10 per cent gelatine solution, and 1200 cc.
of the normal sodium hypochlorite solution prepared as above.
This mixture is heated as rapidly as possible and boiled down
until one-third of the original volume is left. This solution
is then cooled thoroughly with ice and filtered with suction,
first through two layers of toweling and then through one
thickness of ordinary filter paper over cloth, in order to remove
finely divided solid impurities. The solution is then placed
in a precipitating jar, and cooled down thoroughly (0'0) with ice
and salt; 10 cc. of concentrated sulfuric acid for each 100 cc.
of solution are gradually added with constant stirring.
A precipitate of hydrazine sulfate (NH2NH2<.>H2SO4) forms.
The mixture is allowed to stand in the cold for a few hours in order
to complete the precipitation, and is then filtered by suction in the
usual way and washed with cold alcohol. The yield varies from 53 g.
to 58 g. per 1500 cc. of ammonia water (34-37 per cent of the
theoretical amount). The product is perfectly white and crystalline,
and satisfactory for almost any purpose. If an absolutely pure
product is desired, it must be recrystallized from water.
For every 21 g. of crude product, 100 g. of boiling water are used.
If the crude hydrazine is brown, it is advisable to use a little
bone-black. After the mixture has been filtered and cooled to 0'0, 19 g.
of pure white crystals are obtained.