The Filters.
Thinking that a Gooch crucible with a platinum sponge on the bottom in place of asbestus would be desirable for this work one was accordingly made and answered the purpose very satisfactorily. All determinations were made by using such filters. C. E. Munroe (Chem. News, Vol 58, p. 101) has described the preparation of these filters. A platinum Gooch crucible was placed on a filter paper and some ammonium platonic chloride which had been thoroughly washed introduced by suspending it in alcohol and then pouring this into it. The precipitate settles to the bottom forming a uniform layer and the alcohol drains, off through the filter paper. The crucible was then dried slowly in an air-bath. After this it was transferred to a porcelain crucible and slowly heated until decomposition was complete. In this manner a layer of platinum felt is obtained which acts as a very efficient filter. Another layer of double chloride was then decomposed as before so that if there were any imperfections in the first layer they would be covered by the second layer. The surface was smoothed down by means of a glass rod. To prepare a good filter the drying and subsequent heating should be very slow. The heating must not be at too high a temperature, otherwise the felt becomes very compact and is useless for filtering purposes. Pressure produces the same effect. The filters were always treated with strong nitric acid, washed and reheated before being used, but in no case was chlorine detected in the nitric acid after the washing, nor any loss in weight of the crucible. An objection to the use of these crucibles for the purpose named was found in the course of this work, but it will be discussed later. The crucibles were always set in a large weighing-glass, and another weighing-glass containing an equal amount of platinum foil used as a tare, in weighing. This precaution was perhaps unnecessary, but at least it did no harm.