136 The Phosphates of America. 



an atmosphere thickly laden with noxious vapors, it is nevertheless 

 demonstrable that to the extent of the heat liberated in its own 

 condensation, it retards the perfect filtration of the residual vapors, 

 and any benefit accruing from its introduction is wholly dispropor- 

 tionate either to its quantity or its expense. 



The most important point is to cool the gases by draughtage 

 into chambers or flues of sufficient area or length, and where this 

 can be managed economically little more is required, for the fume 

 will quietly subside of itself. In the majority of cases, however, a 

 maximum of condensing work must be accomplished in a minimum 

 of space, and here the better way is to submit the gases to a sort 

 of dry-scrubbing process so as to hasten the deposition of the 

 fluorine compounds. How this is to be done must depend upon the 

 special circumstances in each particular case, but there should al- 

 ways be provided, within a suitable flue, a sufficient number of im- 

 pinging or baffling diaphragms, to momentarily arrest the motion 

 of the gases and divert them into another direction, it being found 

 that the greatest deposition of silica takes place at these eddying 

 points. 



The great bulk of the solid matter being thus early arrested, 

 only the residual vapors now remain to be dealt with, and these are 

 caused to traverse, in an upward direction, one or more water tow- 

 ers or wet scrubbers, simply packed with wood spars, to pass away 

 to the chimney. 



The necessary draught is created by an exhaust-fan of special 

 construction actuated by the mixer engine. It is best fixed be- 

 tween the towers and the chimney, and its power is controlled by 

 a damper just sufficiently to secure a slight "pull in " at the mixer 

 mouth. The den doors are, of course, made as tight as possible to 

 avoid unnecessary dilution of the gases and interference with the 

 efficiency of the fan. 



Gas dilution means reduced condensing efficiency. Yet there 

 have been hosts of failures, due to a total misapprehension of the 

 necessities of the case, and to the impracticable construction or 

 wholly insufficient capacity of the condensing plant. In the erec- 

 tion of the latter two things have to be constantly borne in mind : 

 First, that the evolution of the gas is spasmodic and (especially 

 in the case of hot vitriol) extremely violent when the spasm is on ; 

 and, second, that every chokable part of the apparatus must admit 

 of the readiest possible access. * To provide for the first of these, 

 the plant has to be of ample dimensions, and unless the second be 



