186. The gasometer, suggested for the removal of surplus chlorine (98), might be made by attaching a leaden curtain to a disc of wood to which the suspending chains would then be fastened. A light tub, with wooden hoops, thoroughly soaked with paraffin or ozokerite, would probably answer the purpose.

The chlorine withdrawn from the vats, would, most likely, be contaminated with air, and as its re-use is not of great importance, the apparatus might be simply used to blow air through the ore mass, from below the filter, thus expelling the chlorine through a hose connecting with the hole in the cover of the vat, and conducting out of the room.

In this way there would be no difficulty in the construction of the bell, as either wood or iron could be used. Other methods of blowing air through the ore will readily suggest themselves.

Precipitating vats for gold are sometimes made in the form of rectangular tanks of wood, lined with sheet lead. This vat, though more expensive than a wooden tub, has some advantages. There is no risk of absorption of the gold solution, and it is not necessary to add the precipitant until the vat has received its quota of solution. Then, if the solution contains much free chlorine, a jet of steam may be introduced and the chlorine driven off, after which the precipitation of the gold by means of iron sulphate proceeds better, and with a smaller consumption of the precipitant.

As it is necessary to heat the solution in order to drive off the chlorine in this way, it cannot be thus done so thoroughly in a tarred tub, on account of the softening of the tar. It could, however, be done by blowing air through the solution.