34 § 39. ANALYTICAL MANIPULATION. 



tration, and of the washing of precipitates, promises to be 

 very useful. 



He supports the filter by a hollow cone of thin plat- 

 inum foil in the throat of the funnel, and then rarefies 

 the air in the funnel-tube ; the excess of pressure on the 

 liquid in the filter causes it to flow througli very rapidly, 

 while there is no danger of tearing the paper. 



To make the platinum funnel, a cast of the glass funnel 

 must first be taken. Select a funnel with perfectly smooth 

 and straight sides, ai)d opening at an angle of 60°, fit in 

 it a piece of oiled writing paper in such a manner that it 

 shall touch the glass everywhere, like an ordinary well- 

 fitted filter, and fasten the paper in place with two or 

 three drops of sealing-wax around the rim. Half fill the fun- 

 nel then with gypsum j^aste, into which, before it hardens, 

 a plug of wood is inserted, to serve as a handle. When the 

 gypsum cone has hardened, remove it from the funnel, oil 

 the paper again, and plunge it, with the paper still adher- 

 ing, into a large porcelain crucible filled with another por- 

 tion of gypsum paste ; vfhen tliis mould has hardened, 

 take the cone out and rub off the paper with the fingers. 

 g Now, cut out a piece of thin plat- 



inum foil weighing about 0.154 grm., 

 of the precise shape and size repre- 

 sented in the adjoining figure, vrith a 

 slit running from h to a^ the centre 

 of the circle of which the arc, c c d^ 

 forms a part ; ignite it in the flame of 

 the lamp to make it perfectly flex- 

 ible, lay the gypsum cone on it so that the apex 

 of the cone shall coincide with r/, bring up the edge, a h c?, 

 and press it well against the cone, and then do the same 

 with the edge, ah c ; after fltting the foil to the cone as 

 perfectly as possible with the fingers, put the whole in the 

 mould in the crucible, and revolve the cone back and 

 forth until the platinum has taken the exact shape of the 



