THE CHEMICALS REQUIRED. 31 
showed that it contained traces of nitric acid, the presence <>l' which 
might explain the burning. Nitric acid is one of the most active of 
chemicals and is unstable as well. When heated it readily volatilizes. 
By adding sulphuric acid to water a great amount of heat results. If 
nitric acid be present in the sulphuric acid as an Impurit \ it would be 
far more volatile than under ordinary circumstances The addition 
of the cyanid increases the heat, at the same time causing hydro- 
cyanic-acid gas to be violently thrown off. This gas assists in carry- 
ing off the volatilized nitric acid, which, condensing on the cool, 
moist surfaces presented by the Fruit and leaves of the citrus trees, 
might result in hums or pits. 
In procuring sulphuric acid for Fumigating purposes, only that 
should be purchased which is entirely \'vco of nitric acid, and which 
is guaranteed 66° ( Baum6), or 93 per cent pure. 
Some commercial sulphuric acid on the market meets all the 
requirements of fumigation, while much can be found which does 
not. To enter fully into the reason for this would he out of place in 
this bulletin. All that is necessary is to mention briefly the char- 
acter of the material and processes used by various manufacturers, 
some of whom strive to place a better grade of acid on the market 
than do many others. 
In the manufacture of sulphuric acid, sulphur may be considered 
the basic element. This is obtained from one of two source-, viz, 
from free sulphur, known commercially as brimstone, or from sulphur 
in combination with a metal, as iron or copper pyrites. Brimstone 
is comparatively pure sulphur, containing little or nothing which 
would reduce the grade of the acid manufactured from it. It some- 
times contains a very small quantity of ash. Pure iron pyrites con- 
tains about 53 per cent of sulphur and about 47 per cent of iron. 
Copper pyrites contains much less sulphur. Ordinarily the pyrites 
used in making acid contains small quantities of other elements, as 
arsenic, zinc, lead, etc. To manufacture sulphuric acid, it is neces- 
sary to convert the sulphur into a gas, sulphur dioxid, which is 
brought about by burning the crude product in a retort. The 
sulphur dioxid thus formed is conducted into certain chambers 
where it is mixed with fumes of nitric acid, air, and steam, the 
resulting product being dilute sulphuric acid. Where brimstone is 
used comparatively pure sulphuric acid is formed. When, however, 
pyrites are burned, other (dements present in the ore (as arsenic, etc.) 
are volatilized, pass along with the sulphur dioxid, and are present in 
the crude acid. 
That which concerns us most vitally in fumigating is the presence 
of nitric acid. A much greater proportion of nitric acid becomes 
mixed with the products of combustion from pyrites than from brim- 
stone, resulting in the presence of a larger amount of this undesirable 
