44 HYDROCYANIC-ACID GAS FUMIGATION IX CALIFORNIA. 
WATER AS A FACTOR US FUMIGATION. 
There are several reasons why water should always be employed 
in fumigation. It is very useful in dissolving the potassium cyanid 
and hastening and completing the chemical reaction with the acid. 
A piece of cyanid thrown into a mixture of acid and water immediately 
gives up a portion of its mass in solution. Scarcely has the cyanid 
dissolved when it is partially converted into gas. The heat liberated 
during this process assists in forcing the solution of more cyanid 
which is also partially converted into gas. This continues until 
the chemicals are exhausted and the reaction ceases. 
Potassium sulphate, a solid, is the by-product result ing from the 
reaction by which hydrocyanic-acid gas is produced. Water dissolves 
the potassium sulphate as it forms and prevents it from coating the 
cyanid not yet in solution. In the presence of an insufficient amount 
of water, the potassium sulphate is not completely dissolved, but 
forms a coating on the pieces of cyanid, preventing the sulphuric 
acid from penetrating to it, and thereby retarding, or even in part 
preventing, the reaction. In such cases this undissolved potassium 
sulphate usually solidifies, causing the pots to " freeze. ,? This 
phenomenon always occurs where the formula is 1-1-1, or where 
the same amounts of water, acid, and cyanid are used. On agitating 
the residue by stirring, it is almost always possible to find small 
pieces of undissolved cyanid enveloped in a coating of the potassium 
sulphate. Ordinarily, when the residue is stirred the particles of 
cyanid are removed, to some extent, from this envelope of potassium 
sulphate, allowing some of the unused acid to reach them, and thus 
evolving a small amount of gas without the addition of more acid. 
Under these conditions, however, the reaction is never complete, 
and it is highly desirable, therefore, to add sufficient water at the 
beginning to dissolve all the potassium sulphate. 
Recalling the statements made in discussing the amount of sul- 
phuric acid to use, it is seen that the "congealing" or "freezing" 
of the residue in the generating jars is due to either or both of two 
conditions: (1) An insufficient amount of water to completely dis- 
solve the sulphate of potassium, or (2) a large excess of sulphuric 
acid, whereby the water is rendered less capable of taking into 
solution the same amount of sulphate as it otherwise would. 
Another very important function of the water in the reaction is 
the heat produced by the union of the sulphuric acid and water. 
Potassium cyanid introduced into this heated mixture gives off 
hydrocyanic-acid gas much more quickly and thoroughly" than at a 
lower temperature, and in field work rapid generation of gas is 
essential. 
The action of pure or highly concentrated sulphuric acid on potas- 
sium cvanid results in a verv different chemical reaction than when 
