THE CHEMICALS REQUIRED. 31 



showed that it contained traces of nitric acid, the presence of 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 impurity 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 burns or pits. 



In procuring sulphuric acid for fumigating purposes, only that 

 should be purchased which is entirely free of nitric acid, and which 

 is guaranteed 66° (Baume), or 9.3 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 be 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 sources, ^'iz, 

 from free sulphur, known commercially as brimstone, or from sulphur 

 in combination ^\dth 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 cjuantity 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 cnide 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 elements 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 



