﻿Prof. R. Bunsen on the Washing of Precipitates. 5 



filter. The solution of this problem is very easy : an ordinary 

 glass funnel has only to be so arranged that the filter can be 

 completely adjusted to its sides even to the very apex of the cone. 

 For this purpose a glass funnel is chosen possessing an angle of 

 60°, or as nearly 60° as possible, the walls of which must be 

 completely free from inequalities of every description ; and into it 

 is placed a second funnel made of exceedingly thin platinum-foil, 

 and the sides of which possess exactly the same inclination as 

 those of the glass funnel. An ordinary paper filter is then in- 

 troduced into this compound funnel in the usual manner; when 

 carefully moistened and so adjusted that no air-bubbles are 

 visible between it and the glass, this filter, when filled with a 

 liquid, will support the pressure of an extra atmosphere without 

 ever breaking. 



The platinum funnel is easily made from thin platinum-foil in 

 the following manner : — In the carefully chosen glass funnel is 

 placed a perfectly accurately fitting filter made of writing-paper; 

 this is kept in position by dropping a little melted sealing-wax 

 between its upper edge and the glass ; the paper is next saturated 

 with oil and filled with liquid plaster of paris, and before the 

 mixture solidifies a small wooden handle is placed in the centre. 

 After an hour or so the plaster cone with the adhering paper 

 filter can be withdrawn by means of the handle from the funnel, 

 to which it accurately corresponds. The paper on the outside 

 of the cone is again covered with oil, and the whole carefully 

 inserted into liquid plaster of paris contained in a small crucible 

 4 or 5 centims. in height. After the mixture has solidified, 

 the cone may be easily withdrawn ; the adhering paper filter is 

 then detached, and any small pieces of paper still remaining 

 removed by gently rubbing with the finger. In this manner a 

 solid cone is obtained accurately fitting into a hollow cone, and 

 of which the angle of inclination perfectly corresponds with that 

 of the glass funnel. 



Fig. 1 (Plate I.) represents the cones. By their help the 

 small platinum funnel is made. A piece of platinum (fig. 2 

 shows the natural size) is cut from foil of such a thickness that 

 one square centimetre weighs about 0*3 54 grm., and from the 

 centre a a vertical incision is made by the scissors to the edge 

 cbd. The small piece of foil is next rendered pliable by being 

 heated to redness, and is placed upon the solid cone in such a 

 manner that its centre a touches the apex of the latter; the sides 

 a>b,d are then closely pressed upon the plaster, and the remaining 

 portion of the platinum wrapped as equally and as closely as 

 possible around the cone. On again heating the foil to redness, 

 pressing it once more upon the cone, and inserting the whole 

 into the hollow cone and turning it round once or twice under a 



