66 



SMITH S INTERMEDIATE CHEMISTRY 



will not remove all bacteria. In many cases, small amounts of 

 plum, or alum and lime, are added to the water, and the 

 suspended matter is then allowed to settle, which it does very 

 quickly, in large basins or reservoirs. By this coagulation 

 method (p. 470), all but a few of the bacteria are re- 

 moved. Sometimes the remaining organisms are 

 destroyed by adding a little bleaching powder before 

 the water is distributed (p. 226). 



In the household, the simplest appliance is the 

 Pasteur filter. It consists of a tube of unglazed 

 porcelain, closed at one end (Fig. 34), through the 

 pores of which the water is forced by its own pres- 

 sure. The cylinder (" bougie ") should be cleaned 

 daily with a brush to remove the mud and the or- 

 ganisms which collect in its surface. 



All forms of niters must be cleaned at short in- 

 tervals. If this is not done, the organisms multiply 

 and soon the filter pollutes the water instead of 

 purifying it. 



Filtration does not remove dissolved matter, and 

 therefore does not soften hard water (p. 13). For this latter 

 purpose washing powders are used in the laundry (see p. 390-1). 

 All organisms can be killed by boiling unfiltered water, but the 

 boiling should continue for at least 10 to 15 minutes to be effective. 

 Pure water for chemical purposes is prepared by distillation 

 (Fig. 35). Dissolved solids remain in the flask (or boiler). The 

 steam is condensed by cold water circulating in the jacket of the 

 condenser. Freshly distilled water contains only gases dissolved 

 from the air. If kept in a vessel, however, such water quickly 

 dissolves traces of glass or porcelain. The purest water is made 

 by using a platinum tube in the condenser and a platinum bottle 

 as the receiver. 



Chemical Properties: Stability. As we should infer from 

 the vigor with which its constituents combine, water is a very 



FIG. 34 



