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



turned in a lathe, and then the thin plates sawn off by means of 

 a small hand-saw in the small wooden support shown in fig. 6. 

 The upper surfaces of the plates may afterwards be rendered 

 perfectly even by a coarse file. 



By the aid of these pumice-stone filters many chemical pro- 

 ducts may be made, the preparation of which has hitherto been 

 almost impossible. For the sake of example I take the prepa- 

 ration of pure dry chromic anhydride ; in an hour it is easily 

 possible to filter, wash, and dry crystals of this substance an inch 

 in length. A solution of 2 parts of potassium dichromate in 

 20 parts of water mixed with 10 parts of concentrated sulphuric 

 acid, deposits, after standing about 24 hours, numerous brilliant 

 needles of chromic anhydride. These may be drained from 

 adhering mother-liquor upon the pumice filter by means of the 

 pump, and in a few minutes completely washed by a small quan- 

 tity of fuming nitric acid free from nitrous acid. A covering of 

 tolerably strong sheet copper provided with two arms, as shown 

 in fig. 5, is then placed round the tube; by hanging lamps upon 

 the arms the tube may be readily heated to about 60° or 80° C. ; 

 and by connecting a chloride-of-calcium tube with the upper 

 end of the glass vessel, a current of dry air may be drawn through 

 the apparatus by means of the pump, and thus in a compara- 

 tively short time large and brilliant crystals of chromic anhy- 

 dride, perfectly dry and free from all impurity, may be easily 

 obtained. 



A single pump of the above description costs, including the 

 leaden piping, about 8 thalers (24 shillings); and experience 

 has shown that five or six are amply sufficient for a laboratory 

 of fifty or sixty students. The apparatus, as may readily be 

 seen, can be applied in the operation of evaporating in vacuo; 

 if, however, circumstances will not permit of its being adapted to 

 this purpose, then a fall of 10 or 15 feet is sufficient to filter a 

 precipitate according to the above described method, and so far to 

 dry it that it can be immediately ignited in the crucible. It is 

 therefore not absolutely necessary to employ an air-pump in this 

 process of filtration ; any apparatus producing a difference of 

 pressure amounting to a quarter of an atmosphere is sufficient. 

 The simple arrangement represented in fig. 7 is very useful, and 

 is frequently employed in my laboratory. It consists of two 

 equal-sized bottles, a and a, } of from 2 to 4 litres capacity, each 

 of which is provided near the bottom with a small stopcock de- 

 signed to regulate the flow of water. Suppose a filled with 

 water and placed upon a shelf as high above the ground as pos- 

 sible and a! placed empty on the floor, and the two stopcocks 

 connected by means of caoutchouc tubing c, then on allowing 

 water to flow down the tube the air in the upper bottle be- 



