OF ARTS AND SCIENCES. 



117 



In the upper chamber, and resting on the plate, is placed coarsely 

 powdered sulphide of iron, which is put in through the bung B. 

 Through the plate M, and on two lines at right angles to each other, 

 there are cut radial slits, ^ an inch wide, which are guarded by four 

 iron arms. These arms are attached to the agitator shaft S, and move 

 over the surface of the plate, alternately covering and uncovering the 

 slits, when the handle E is turned. To the lower end of the same 

 shaft is fastened the agitator O, which is turned simultaneously with 

 the arms just mentioned. After the apparatus has been charged, it is 

 evident that by turning the handle the sulphide of iron may be sifted 

 down at pleasure into the acid water below ; and the handles and arms 

 are so disposed that when the bungs are uncovered by the handles the 

 slits are covered by the arms. From the generator, the gas passes 

 through the washer R, whose relations to the apparatus, as well as that 

 of the pressure gauge, are evident from the figure. The generator is 

 lined with lead, and the rest of the apparatus, including the bell-metal 

 plate M, with pure tin. The lead lining of the generator is seamless 

 and very heavy, and all the cocks are lined with block tin tubing. Of 

 the various valves, bungs, and stuffing boxes indicated in the figure, it 

 is unnecessary to speak in detail. It is sufficient to say that they are 

 of excellent workmanship, and during a year's trial have kejit perfectly 

 tight. The charging bung, B, however, is closed by a safety cap of 

 peculiar construction, which deserves special mention, because it insures 

 the safety of the apparatus. The cap is represented by Fig. 3, and a 

 section is given in Fig. 4. It will be seen by the last that the escape 

 of the compressed gas from the generator through the apertures d is 

 only prevented by a thin disk a, which is shown in detail by Fig. 5. 



Fig. 3. 



Fig. 5. 



FifT. 4. 



This disk is made of two thin plates : the lower one, which comes in 

 contact with the acid spray, is of lead, and the upper one of silvered 

 copper, whose thickness is so adjusted that it must be at once ruptured 

 if the pressure in the apparatus should become unduly great. 



