THE EMIGRANT'S HAND-BOOK. 443 



purified water. Then, to clean the filtering apparatus, 

 reverse the process, and fill the division b with unpurified 

 water ; it will pass through the aperture in the partition 

 b, and descend through the cloth, the sjmd, and perforated 

 plate c, rising in the division a, and carrying with it all 

 the impurities, which may be drawn off by a pipe fixed in 

 the bottom. 



CHEMICAL TESTS FOR EXAMINING WATER. 



When good water is first taken up, it is perfectly clear 

 and colorless ; but the examiner should not be satisfied 

 with holding up a glassful between his eye and the light ; 

 he should also pour some into a deep ale glass, into which 

 he should look downward, when the slightest tinge from 

 extraneous substances will appear. 



If the water be discolored in any manner, it is usually 

 owing to some impurity mechanically suspended in it, 

 and which may be removed by rest and filtration, in the 

 manner already described : but the impurities may like- 

 wise be dissolved in the water, in which case they will 

 pass through the filter, which has no power of separating 

 them. 



To determine whether the water contains lime in any 

 form, the oxalic acid should be employed, as the best test 

 for this earth. All waters containing lime are more or 

 less injurious to health, affecting the kidneys if they are 

 drank for any continuance. Some of the springs about 

 London contain a great deal of sulphate of lime, and are 

 unfit for washing, and would be unhealthy to drink. To 

 explain the principle upon which oxalic acid proves the 

 presence of lime, it must be observed, that lime has a 

 stronger attraction for the oxalic than it has for any other 

 acid ; therefore it will quit whatever acid it may be com- 

 bined with in a dissolved state, and unite to the oxalic, 

 forming an oxalate of lime, which, being insoluble in 



