173. Filters for dumping vats are made with a bed of twigs, in place of gravel, and over the twigs is a piece of cloth, which is held close to the inner surface of the staves by a wooden hoop. The objection to burlap or other cloth is, that it is speedily destroyed by chlorine gas. Attempts have been made to preserve it, by soaking it in coal tar and asphalt, but without much success. It occurred to me to use asbestos cloth, but I found none that was not too closely woven. Clay tiles, of the kind known as "biscuit ware," have been suggested for filters, and it may be that basket work, made of rattan, would answer, especially if soaked in petroleum, or ozokerite.