132 HEALTH AND DISEASE 



ledge will be obvious when it is remembered that the hardness caused by 

 the carbonate of lime may be got rid of by a comparatively simple process 

 (the addition of a small quantity of lime, and precipitation of the carbonate. 

 See page 123), while that caused by the presence of sulphate, chloride, 

 and nitrate of lime cannot be removed. The test, however, for chlorine, 

 sulphuric acid, and nitric acid would be rather beyond the powers of the 

 amateur. 



The presence of organic bodies is generally considered to render water 

 unfit for use, but a great deal must depend upon the nature of the organic 

 matter, whether animal or vegetable, and also on the state of decomposition 

 which has been reached. Samples of water which have been found to contain 

 an enormous quantity of vegetable matter have been taken by animals with 

 perfect impunity, doubtless on account of the matter being of vegetable 

 origin, and not having undergone decomposition. Microscopic examination 

 of water, especially water which deposits various impurities after rest, is the 

 only satisfactory method of discovering the character of the impurities. A 

 further step in the same direction is inoculation of nutritive media, such as 

 pure gelatine and other similar subtances used by the bacteriologist, but 

 this, as well as the use of the microscope, must necessarily be left in the 

 hands of the expert. 



EXAMINATION OF AIR 



In an absolutely pure state, such as could only be secured by the 

 admixture of the two essential constituents, oxygen and nitrogen, in proper 

 proportion, the air does not exist in nature. The purest air contains a 

 minute quantity of carbonic acid and a certain amount of water vapour, 

 with traces of ammonia and ozone, varying in amount according to the 

 situation, as well as organic and mineral particles. Of the constituents 

 of healthy atmosphere, oxygen deserves the chief consideration as a power- 

 ful agent in the destruction of various impurities. Roughly, its proportion 

 may be taken as one-fifth, while nitrogen constitutes the greater part of 

 the remaining four-fifths, exercising apparently its chief function, that of 

 diluting the oxygen and modifying its stimulant action. Animal life is 

 not sustained by nitrogen, as it is a non-respirable gas; it is destroyed by 

 oxygen by being carried on too rapidly — the animal under the violently 

 stimulating, exciting influence may be said to live the whole of its life 

 in a short space of time. It is only, therefore, by the combination of the 

 two elements that a respirable atmosphere, capable of supporting life for 

 its normal period, is obtained. 



Air is distinguished by its almost unlimited capacity for absorbing 



