344 



PROCEEDINGS OF THE AMERICAN ACADEMY 



Fi-. 1. 



Fig. 2. 



First, white, silky, anhydrous asbestus is scraped to a fine short 

 down with an ordinary knife-blade, boiled with hydrochloric acid to 

 remove traces of iron or other soluble matter, washed by decantation 

 and set aside for usd. 



Secondly, a platinum crucible of ordinary size, 

 preferably of the broad low pattern (Fig. 1), is 

 chosen, and the bottom (Fig. 2) perforated with 

 fine holes (the more numerous and the finer, the 

 better) by means of a steel point ; or, better 

 still, the bottom may be made of fine platinum 

 gauze. Next, a Bunsen funnel of the proper size is 

 selected, and — following Munroe's plan for holding his 

 porous cones — over the top a short piece of rubber tubing 

 is stretched and drawn down until the portion above the 

 funnel arranges itself at right angles to the direction of the 

 stem. AVithin the opening in the rubber, the per- 

 forated crucible is fitted as shown in figure 3, and 

 the funnel is connected with the receiver of a 

 Bunsen pump or other exhausting apparatus in 

 the ordinary manner. 



To make the asbestus felt, the pressure of the 

 pump is applied, and a little of the asbestus pre- 

 pared as described, and suspended in water, is 

 poured into the crucible. The rubber and the 

 crucible are held together by the pressure of the 

 vacuum-pump with sufficient force to make an 

 air-tight joint ; the water is drawn through, and 

 the asbestus is deposited almost instantly in a 

 close compact layer upon the perforated bottom ; 

 more asbestus (if necessary) in suspension as before being poured upon 

 the first, until the layer becomes sufficiently thick for the purpose for 

 which it is intended. Finally, a little distilled water is drawn through 

 the apparatus to wash away any filaments that might cling to the 

 inider side, and the filter is ready for use : the whole process occupy- 

 ing less time than is necessary to fold and fit an ordinary paper-filter 

 to a funnel. 



To prepare the filter for the estimation of a precipitate, the crucible 

 with the felt undisturbed is removed from the funnel and ignited. In 

 case the precipitate, to be subsequently collected, must be heated to a 

 very high temperature for a long time, it is better to enclose the per- 

 forated crucible with its felt within another crucible ; because, in such 



