§ 39. FILTRATIOX. 31 



ber to subtract the weight of 1']^ or V\^ filter-ash from 

 the weight of the ignited residue instead of 1 as usual. 



• FILTRATION. 



39. a. — Solid particles are separated from the liquids 

 with w^hich they may be mixed by the process of filtra- 

 tion, referred to in the preceding paragraph, which con- 

 sists simply in passing the liquid through porous vmsized 

 paper, that intercepts the solid. 



. Paper, already cut in convenient sizes, can be had of 

 apparatus dealers. For quantitative purposes, filters of 

 Swedish i^aper should be used, or common white filters 

 that have been washed in dilute acid ; to wash filters, 

 pour over them, in layers of moderate thickness in a large 

 evaporating dish, a mixture of one part of hydrochloric 

 acid and nine parts of water ; digest for several hours at 

 a moderate temperature, wash with distilled water by de- 

 cantation until the washings no longer redden litmus, 

 transfer the bunches of paper to blotting paper, and leave 

 them undisturbed until the filters can be separated from 

 each other Avithout being torn. These washed filters 

 are more suitable for filtration by Bunsen's process than 

 those of Swedish paper, as they are stronger and less lia- 

 ble to be torn. 



To make the filter, fold the circular piece of paper twice 

 in directions at right angles to each other, and through 

 the centre ; open the quadrant thus formed in such a man- 

 ner as to make a conical cavity, put it in a glass funnel, 

 which should be at least 3-5 millimetres larger than the 

 filter, wet the latter with a Httle water from the washing- 

 bottle, and press it closely against the glass throughout 

 with the finger. 



The filter should never be filled Avith the liquid to 

 within less than 6 mm. of the top, and should not ordi- 

 narily be much more than half filled with the precipitate 

 when the liquid has drained off. 



