OF ARTS AND SCIENCES. ItiJ 



in the bottle, and the beaker having been placed on the stand, we 

 begin by attaching the paper disk to the base of the rose, moistening 

 it for the pnrpose with water, and forcing it with the thumb against 

 the perforated plate until the adhesion is complete at every point. It 

 is important that the disk should not overlap the edge of the plate ; and, 

 if the plate has been made as described, it will adhere tightly without 

 so doing. The compression cock is then slowly opened ; and, as soon 

 as the sound indicates that the air is being sucked through the filter, 

 the base of the rose is quickly sunk under the liquid, and constantly 

 lowered by the rack and pinion movement, as fast as the liquid is drawn 

 off. The supernatant liquid may thus be decanted until the rose is 

 close down to the precipitate ; but care should be taken at this stage 

 not to push the process too far, lest the filter should become clogged 

 by the adhesion of solid particles, as would be the case, if it comes too 

 near the level of the precipitate. When the safe level has been 

 reached, the arm of the stand is suddenly pushed upwards, while the 

 rose is inverted and supported as above described. The beaker is now 

 filled up with hot water ; and, after the precipitate has subsided, the 

 wash water is drawn off as before, and so repeatedly as often as may 

 be necessary. It is important, however, to carefully watch the filter, 

 and every time before immersing the rose to see that the paper is 

 moist and firmly adhering to the perforated plate. It is further essen- 

 tial that the suction should not be for a moment interrupted while the 

 filter is immersed. This condition is secured by the simple rubber 

 joint we have described ; for, while when the rose is inverted the con- 

 nection with the bottle is closed, the moment it is turned down the 

 connection is opened, and the filter begins to draw. If, however, by 

 any accident the filter should slip, the operator when on his guard can 

 avoid loss of material by quickly opening the nipper tap, and relieving 

 the pressure until the rose can be withdrawn and washed out. The 

 filter can then be fished out with a glass I'od, washed off and replaced. 

 A good strong filter will bear quite rough treatment ; and, if in the pro- 

 cess it becomes clogged, it can be taken off when the rose is inverted, 

 and the adhering precipitate washed back into the beaker. When re- 

 placed, the paper thus cleansed often filters as rapidly as before. If, as 

 is sometimes the case, the paper becomes hopelessly clogged, no great 

 loss of accuracy is suffered by using a second or even a third filter. 

 Of course, they must all be dried, and weighed either with the precipi- 

 tate or apart, as most convenient. 



The precipitate, having been thus washed, is next to be transferred 

 to the crucible in which it is to be weighed ; and, to hold five grammes 



