The Phosphates of America. 123 



The upper and lower halves are held together by hinged bolts in 

 slots cut in the flange of each section. The upper half is hinged 

 to the lower at one side and is easily raised so as to give free 

 access to the interior of the mill for examination and the replacing 

 of worn parts. The casing and the three pedestal bearings for 

 the shaft are seated upon a heavy bed-plate as indicated in the illus- 

 trations. Being a balanced machine it does not require elaborate 

 or expensive foundations. 



The difficulty of estimating the exact cost of grinding phos- 

 phates to a fineness of 70 or 80 mesh, either by the methods we 

 have thus described or any others now in use and perhaps equally 

 good, is naturally very considerable, since so much must, perforce, 

 depend upon the nature of the material itself. We have seen it va- 

 riously estimated at from 50 cents to $2 per ton, and have even met 

 those who claim to be able to do the needful work for less than 

 the first figure. As a matter of sober fact, however, we have 

 found that in practice, when breakages, repairs and general wear 

 and tear are taken into account, $1.50 per ton is more like the 

 proper figure, and we therefore usually adopt it as a fair basis 

 for calculations. 



The operation of grinding having been satisfactorily performed, 

 the phosphate is submitted to complete analysis and, its chemical 

 composition being thus known, is finally conveyed to the mixer. 



The mixing together of the raw materials in the proportions 

 determined by proper computation is performed in a commodious 

 shed, of which the annexed drawing will convey the necessary under- 

 standing. It must be near to the sulphuric-acid chambers, and 

 directly connected with a high shaft or chimney and a condensing 

 apparatus or scrubber ; the latter for absoibing the noxious fumes 

 set free by the decomposing mass. 



A strong brickwork shell with a good foundation is built in 

 the centre of the shed. This shell is divided off into from six to 

 twelve chambers or "dens" some 15 feet square and 20 feet high, 

 each of which must communicate, by means of a good-sized flue, 

 with the scrubber and factory chimney. 



The air-tight iron doors of the "dens" must slide easily back- 

 ward or forward when the superphosphate has become dry enough 

 to be dug out. 



The tops of the "dens" are fitted with mixers of cylindrical 

 shape about 10 feet long, 3 feet in diameter and 4 feet high. 

 The mixer may be constructed of wood lined with sheet lead, or 



