30 DIGESTION AND FOOD. 



animals, which constantly presents itself in impure waters. 

 These things live in water containing decomposing animal 

 and vegetable matter; and it is this matter which is injurious. 

 So that, although the living creatures themselves are not in- 

 jurious, the water they live in is." 



On the subj-ect of organic impurities > Dr Lankester says:, 

 "Organic matters may be dissolved in* water, and then they 

 cannot be found by the microscope. The chemist estimates 

 these by the quantity of nitrogen which he obtains from the 

 deposit of water which has been evaporated ; but it is very 

 difficult to estimate this form of impurity. I have found the 

 permanganates of potash and soda a very good rough test for 

 ascertaining the presence of this dissolved matter. Per- 

 manganic acid and the permanganates contain large quantities 

 of oxygen; and, when they are brought in contact with 

 organic matters, they lose their oxygen and become changed 

 in colour. If you take permanganate of soda, which is sold 

 in the shops under the name of Condy's Disinfecting Fluid, 

 and put it into pure water, it produces first a deep violet, and 

 afterwards a beautiful permanent red colour. If the water, 

 however, contains organic matters, the red colour soon dis- 

 appears, and in proportion to the quantity of organic matter 

 will be its decolorizing agency. Now, if you take a series of 

 waters of different degrees of impurity, you will find that the 

 water which has least impurity retains the most colour. I 

 have tried this in so many instances with a perfectly success- 

 ful result, that I can confidently recommend it as a test for 

 ascertaining the relative quantities of impurity in water. 

 The same test has been applied by Dr Angus Smith for ascer- 

 taining the organic impurity of the atmosphere; and by this 

 means he has arrived at some very interesting results. It 

 should, however, be recollected that many other impurities 

 besides those of organic origin mav exist in the atmosphere 



