Physical Properties of Water 277 



to dryness and, when cool, weigh the dish again. The 

 gain in weight should be 2-5 g., if the mixing was 

 thoroughly done. 



A check on this result may be performed if the 

 filter-paper before use be weighed after drying for a 

 quarter of an hour in a steam -oven. In this case, all 

 the undissolved portion must be washed out from the 

 beaker in which the dissolving took place into the filter, 

 washed several times by pouring small quantities of 

 distilled water through the paper, and the filter and its 

 contents weighed after drying in a steam-oven for a 

 quarter of an hour. To make sure that all water has 

 been removed, the paper should be put back into the 

 oven for five minutes and again weighed. Loss in 

 weight means that more water has been evaporated, 

 and further drying should be resorted to until the results 

 of two consecutive weighings agree. The weight of 

 dry filter and residue, less that of the filter, is that of the 

 quantity of undissolved matter. 



191. Should the substance held in suspension con- 

 sist of large or heavy particles, an almost complete 

 separation of the liquid and the suspended substance 

 may often be made by " decantation ". The vessel con- 

 taining the liquid is allowed to stand in a quiet place 

 for some time so that the suspended matter settles at 

 the bottom of the vessel. The liquid is then poured, 

 care being taken to agitate it as little as possible, into 

 another vessel. A glass rod held against the edge of 

 the pouring vessel at the point where the liquid is 

 escaping serves as a guide for the direction of the 

 liquid into the lower vessel and prevents it from running 

 back down the side of the pouring vessel. 



192. To discover whether a given substance is or 

 is not soluble in water is a simple matter. 



