226 BACTERIA, YEASTS, AND MOLDS 



Seltser water, etc. Such beverages are not bacteria free, 

 and a study of a large variety has shown that occasion- 

 ally the number of bacteria they contain is considerable. 

 Artificial aeration, that is, charging the water with carbon 

 dioxide, does not at once destroy germs, and if the water 

 thus charged contained disease germs at the outset, the 

 water is not rendered any safer than it was before aera- 

 tion. Such artificially prepared waters, therefore, are, 

 while fresh, no safer than the original water from which 

 they are made. After they have stood for a few weeks 

 the disease germs seem to die and the water becomes 

 wholesome. The, naturally aerated waters are, so far as 

 known, never likely to be impregnated with disease germs. 

 We may then conclude that naturally aerated water is safe 

 from disease bacteria, and that other forms of aerated 

 water are practically safe if they are not too fresh. In 

 general, such waters are, therefore, more reliable than 

 drinking water which has an opportunity for sewage 

 contamination. 



ice. The question has been raised in the last few years 

 whether ice made from sewage-contaminated water is safe 

 to use for cooling drinking water. Typhoid bacilli are not 

 killed by freezing, and it has been claimed, therefore, that 

 such ice is as dangerous as water. A more careful study 

 of the subject has shown, however, that although the 

 bacilli are not killed by simple freezing, they are mostly 

 rendered harmless if they remain frozen in ice for several 

 weeks. Ice harvested in the winter is therefore safe to 

 use the following summer. This statement applies to 

 clear ice, but not to snow ice sometimes found on the 

 surface of frozen ponds. 



