KNAPP'S CHEMICAL TECHXOLOGT. 285 
Fig. 97. 
which at the same time answer the purpose of coolers. 
The syphon a b c is let into a wide tube d d, which is sup- 
plied with a current of cold water [below] through e. The 
water after becoming warm, flows off at g. The cooling 
is therefore effected by surrounding the hot acid in b c with 
a current of cold water passing in an opposite direction, the 
effect of which is very much increased by making the longer 
limb in four distinct tubes, as in fig. 9S. To fill the syphon 
Fig. 98. 
the cock n is stopped, and acid is poured first into o, and 
then into 6 till it runs down into the retort through b a. 
When and 6 are now closed, and n opened, the syphon 
comes into play.'' 
The exiracliou of common ?alt is next treated of, imme- 
