PREPARA TION OF GASES
77
difficulty; as with these proportions, if the gas is ‘rushing,’
and the flame be lowered, the flow does not diminish for a
minute or two. Hence the probable behaviour must be anti¬
cipated^ as it were. This needs experience, and the beginner
should make his gas cautiously till he has gained it, which
will only take him rather more time to fill his bag. Steadi¬
ness of action can, however, be attained by further adding to
the mixture common salt, for which hint I am indebted to
Mr. E. Holland. Take, say,
Chlorate of potash ... 2 lbs.
Manganese oxide .... £ lb.
Salt ...... 6 ounces.
In this mixture the chlorate is to be powdered as well as
the other ingredients, and the evolution of gas will answer
almost instantly to the lowering of the flame, and thus be
under perfect control. This is therefore the mixture I recom¬
mend : chlorate, 8 parts by weight ; manganese, 2 parts ; salt
1^ parts.
When chlorate is used with manganese alone, it must
not be powdered, but used in the rough crystals ; and 1 lb. of
the chlorate—the only active ingredient—must be allowed
for 4 to 4J cubic feet of oxygen. Powdered chlorate, without
salt, is liable to cause violent rushes. It may also be noted
that finely granulated manganese is better than powdered, as
not choking the delivery-tube at all ; but it is seldom easy to
procure, and the powder need cause no inconvenience.
Using the necessary caution, it often happens that when
the charge is about half exhausted the evolution of gas stops
for a while, and even the full flame fails to start it again for
some minutes. It will resume, however, and during this
second stage there is little need of caution for fear of a rush ;
the full heat is generally needed to the end. For its thorough
manageability, however, I strongly recommend the salt mix¬
ture.