THE MICROSCOPE AS DETECTIVE 



The impuriticfi likely to be present in water make 

 such a formidable list that we can only mention 

 a small number. There may be various mineral 

 substances, such as lime, sand or clay; vegetable 

 substances, starch grains, fragments of wood, pieces 

 of decayed leaves and the like or there may be hair, 

 scales of fish, etc. The impurities may be living 

 plants, of which the most likely to be found are 

 bacteria, diatoms, desmids and Vclvox, amongst 

 plants, and rotifers, Vorticella, Paramoecium, 

 Amoeba, also portions of tapeworms and their eggs 

 amongst animals. These creatures are all described 

 elsewhere so we need not dwell on their peculiarities 

 here. 



In addition to all of the above and many more 

 not mentioned there are four metals commonly found 

 in impure water, either in small solid particles, or 

 in the form of one of their compounds soluble in 

 water. The metals are iron, copper, zinc and lead. 

 Very simple chemical tests will show whether they 

 are present or not. To a drop of the water add 

 a very minute quantity of hydrochloric acid and of 

 potassium ferrocyanide solution. When iron is 

 present the solution will turn blue ; in the presence 

 of copper it will become bronze coloured, whilst 

 zinc turns it a milky white. To detect lead, take 

 another drop of water and add a very small quantity 

 of potassium chromate solution. If the suspected 

 impurity is present the solution turns yellow. All 

 the chemicals for these simple tests may be obtained 

 at any chemists and there is this great advantage 



147 



