48 



PRINCIPLES OF CHEMISTRY 



application, but evidently it is not pure in a chemical sense. A 

 chemically pure water is necessary not only for scientific purposes, as 

 an independent substance having constant and definite properties, and 

 as the chief component of all forms of water which play such an impor- 

 tant part in nature, but also for many practical purposes for instance, 

 in photography and in the preparation of medicines because many 

 properties of substances in solution are changed by the impurities of 

 natural waters. Water is usually purified by distillation, because the 

 solid substances in solution are not transformed into vapours in this 

 process. Such distilled water is prepared by chemists and in labora- 

 tories by boiling water in closed metallic boilers or stills, and causing 

 the steam produced to pass into a condenser that is, through tubes 

 (which should be made of tin, or, at all events, tinned, as water and its 

 impurities do not act on tin) surrounded by cold water, and in which 

 the steam, being cooled, condenses into water which is collected 7 in a 



colour it destroys much of the organic substances it contains. It is especially salutary 

 to add a small quantity of Condy's fluid to impure water in times of epidemic. 



The presence in water of one gram per litre, or 1,000 grams per cubic metre, of any 

 substance whatsoever renders it unfit and even injurious for consumption by animals, 

 and this whether organic or mineral matter predominate. The presence of 1 p.c. of 

 chlorides makes water quite salt, and produces thirst instead of assuaging it. The 

 presence of magnesium salts is most unpleasant ; they have a disagreeable bitter taste, 

 and in fact impart to sea water its peculiar taste. A large amount of nitrates is only 

 found in impure water, and is usually injurious, as they may indicate the presence of 

 decomposing organic matter. 



7 Distilled water may be prepared, or distillation in general carried on, either in a 



FIG. 4. Distillation by means of a metallic still. The liquid in C is heated by the fire F. The 

 vapours rise through the head A and pass by the tube T to the worm S placed in a vessel R, 

 through which a current of cold water flows by means of the tubes D and P. 



