MANUFACTURE OF PHOSPHORIC ACID. 33 
for coke, this coal being added in sufficient quantity to supply 10 per 
cent of fixed carbon to the mass. After a number of tests, however, 
it was found that 14 per cent of coke was ample to give the proper 
reducing reaction and this amount was used in the three other 
batches of briquets where higher proportions of silica were employed. 
In these experiments the same type of small injector gas furnace 
was used as that employed in the preliminary work where pure mix- 
tures of tricalcium phosphate, silica, and carbon were smelted, but 
in order to obtain somewhat higher temperatures than previously 
employed, the air for the combustion of gas was passed through a 
steel coil heated by two Bunsen burners. The air was thus pre- 
heated to a temperature of 250° C. This apparatus is shown in 
Plate III. 
When the furnace had been brought up to a temperature of 
about 1,575° C. a single briquet was dropped into an open graphite 
crucible and the heating continued from 20 to 50 minutes, readings 
being made at intervals of 5 to 10 minutes with an optical pyro- 
meter. Evolution of phosphoric acid usually began about 3 minutes 
after the introduction of the briquet and at the expiration of 10 
minutes the fumes were copiously evolved. The slags produced were 
poured upon a hollowed-out carborundum brick and after cooling 
were ground to 80 mesh and the phosphoric acid remaining therein 
determined by analysis, that which was evolved being calculated by 
difference. The results of these experiments are shown in detail in 
Table 18. 
