FILTRATION. 63 



placed inside the funnel ; the mixture of salt crystals and mother- 

 liquor is poured into the hollow paper cone, when the liquor runs 

 slowly through the pores of the paper, and drops out clear and 

 bright at the end of the funnel, whilst the crystals of salt remain 

 upon the paper. When all the liquid has run away, the crystals 

 may be scraped off the paper on to a 

 plate, and left to dry in the air. Fig. 

 37 represents a method of plaiting or 

 folding the paper which furnishes a 

 filter, through which the fluid passes 

 much more rapidly than through the 

 plain cone. 



Sea- water contains many substances 

 dissolved in it besides common salt ; 

 these communicate the bitter nauseous 

 taste observable in sea-water in addi- 

 tion to the saline taste of the salt. 

 These impurities and other substances 



for the most part remain dissolved in pi 37^ pitted Filter 

 the mother-liquor, so that the salt 

 crystals separated as above are free from the bitter taste, and only 

 taste of pure salt. 



In some countries salt is largely manufactured from sea-water 

 by penning up a large quantity of water in a shallow reservoir ; 

 the heat of the sun gradually evaporates the water, so that by and 

 by a very strong brine is produced. If this is now boiled down 

 by artificial heat with continuous stirring, fine crystals of salt 

 separate, which are " fished " out, slightly sprinkled with water to 

 wash away the small quantity of adhering mother-liquor, and finally 

 dried for table use. If, on the other hand, the evaporation is 

 continued by the natural heat of the sun in the reservoir, larger 

 crystals of bay salt are formed. 



On a hot summer's day this process may be seen going on, on a 

 small scale, among the rocks at low water at the seaside ; shallow 

 puddles of sea-water left in the morning as the tide goes down 

 often become sufficiently evaporated by the heat of the sun before 

 the tide rises again to give perceptible films of crystallised salt at 

 their edges ; or they may even dry up almost entirely, giving a 

 distinct layer of crystals of salt in the hollow where the water origi- 

 nally lodged. In all probability natural rock salt was produced in 

 byegone years in this sort of way. 



The Dead Sea is a lake into which river water flows (the Jordan), 

 but which has no outlet, all the water brought in by the river 

 becoming evaporated by the heat of the sun. As the river water 



