172 CONTAMINATION OF WATER. 



of others. There should be a little dust sprinkled in each day, 

 and occasionally some " chlorid of lime," or sulfate of iron. 



Typhoid fever is now known to be usually caused by 

 drinking-water. The dejecta of some one who has had the 

 disease find their way into the source of the drinking-water. 

 In many cases this has been clearly proved. Of course the 

 dejecta of all such patients should be either destroyed or 

 thoroughly disinfected. 



Although bacteria will not develop in a cold place, they 

 are not killed in the water when it turns to ice, as was for- 

 merly supposed. Further, ice, in forming, does not throw out 

 all the impurities, as was formerly stated. So it is not safe 

 to drink water formed from melted ice unless the water of 

 which that ice was made was good water. The ice taken 

 from ponds is not safe. If ice is made artificially from suit- 

 able drinking-water, of course the melted product will be 

 essentially unchanged so far as the composition is concerned. 

 Water may be cooled by placing any ice around it, and we 

 may have the desired temperature without any admixture of 

 a dangerous element. 



When one cannot get good drinking-water, or when away 

 from home where the water is of doubtful purity, it is better 

 to boil the water before using it, either as a drink or in pre- 

 parations of food that are not to be thoroughly cooked. It 

 seems to be proved that it is better to heat the water twice to 

 near the boiling-point than to boil hard once only. The first 

 heating may start the resistant germs into more active life, 

 causing them to sprout (so to speak), and a second heating 

 several hours later may easily kill them ; whereas it has been 

 proved that one hard boiling will not always kill the germs. 



Water which has been boiled becomes a better medium 

 than it was before for the growth of bacteria which may after- 

 ward get into it, or whose spores may have escaped death in 



