76 MANUFACTURE OF FERTILIZING MATERIALS 



acid, which rotted the bags, besides the mixture of 

 sulphuric acid and hydrochloric acid attacks the 

 metal. 



The gases formed in the superphosphate "dens" 

 cannot be allowed to escape into the atmosphere 

 without being purified, in consequence of their bad 

 smell and corrosive action. They are generally 

 passed through a wash tower by means of a fan. 

 The fans should be rather powerful, so that the 

 amount of air drawn into the "den" during dis- 

 charge is sufficient to allow the laborers to empty 

 the "den" under good conditions. 



