THE MICROSCOPE AS DETECTIVE 



reward for the trouble taken. A drop of strong 

 nitric acid, which must not be allowed to come in 

 contact with any part of the microscope or with 

 one's hands or clothes, will turn the crystals a rich 

 orange colour. The crystals are composed of a 

 substance called piperine. 



Everyone knows of the importance of pure water 

 for drinking purposes but the word pure in this 

 case is used in a very wide sense, for the only really 

 pure water is distilled water and it would not form 

 a very good beverage. Although ordinary tap water 

 may contain a number of impurities it is not easy 

 to see them without taking a little trouble. If our 

 tap water is so contaminated that a drop, examined 

 at random, shows us all manner of solid matter 

 floating in the water there must be something 

 seriously wrong. Those who are engaged in testing 

 water under the microscope, first take a big jug 

 full of the water and allow it to stand for twenty- 

 four hours, covering it the while to keep out dust. 

 At the end of this time most of the water is care- 

 fully drawn off, from the top, with a siphon and 

 the remainder, after stirring is poured into a conical 

 glass and allowed to stand for a further twelve 

 hours. Then the upper portion of the water is 

 again siphoned off and a little of the remainder, 

 which is left in the point of the conical glass is 

 drawn by suction into a special kind of glass tube, 

 called a pipette. This final sample contains all the 

 solid matter which settles to the bottom of the water 

 after standing for thirty-six hours. 



146 



