678 THE POPULAR SCIENCE MONTHLY. 



repellent in the thouglit that we are liable to unwittingly consume, in 

 our drinking-water, as we do in nmch of our uncooked food, such num- 

 bers of living tilings, l>ut this feeling is largely due to the wholly un- 

 justifiable disposition which many persons display to class them among 

 "bugs" and "worms." Nobody thinks of considering the consump- 

 tion of fresh fruits and vegetables as anything uncanny. And yet all 

 the vegetables and fruits which we commonly use as foods are really 

 made up of vast aggregates of tiny living organisms called cells, each 

 one of which is the analogue of the single organisms called bacteria, 

 and under ordinary conditions one is just as little harmful as the other. 

 The leaves and fi-uits of some plants are exceedingly poisonous, and 

 yet he who should on that account decline to eat lettuce or peaches 

 would be justly reckoned among Nature's weaklings. The air we 

 breathe in inhabited regions always contains considerable numbers of 

 bacteria, but they are for the most part harrnless. 



We have learned a great deal about these, our invisible friends the 

 bacteria, within the past few years ; and as that knowledge has grown, 

 we have found out that lurking among them are a few species, not 

 friends but our most inveterate foes, producing disease and even death. 

 The fact is that, under ordinary favorable sanitary conditions, the bac- 

 teria which we are liable to breathe or consume are as harmless as so 

 much air. But if we insist upon drinking dirty water or breathing 

 filthy air, wc increase, as we deserve to do, our risk of coming under 

 the influence of the baneful forms. 



There are a few diseases common among us, the most important of 

 which are consumption and typhoid fever, which are caused by the 

 presence and action in the body of certain well-defined and well-known 

 species of bacteria. These diseases never occur except under the in- 

 fluence of these particular forms of germs. And the reason why con- 

 sumption and typhoid fever continually occur is because certain of us 

 get some of these bacteria in the living condition into our bodies, 

 where they grow and induce the disease. All persons are not alike 

 susceptible to the action of these bacteria, naturally or at all times, so 

 that they doubtless not infrequently gain access to our bodies without 

 producing ill effects. Now eveiy intelligent' person knows, or ought 

 to know, that water polluted with sewage is not a proper thing to 

 drink ; and, while there may be other causes which render it uuAvhole- 

 some, the cause which we know most about is the presence of certain 

 forms of disease-producing bacteria. This knowledge it is which has 

 led to the construction for large towns of expensive systems of watcr- 

 sup))ly, whose reservoirs are situated at considerable distances, where, 

 presumably, no sewage contamination is possible. If we can be certain 

 that4he water from our city supplies can not contain sewage or human 

 or animal excretions of any kind, we are pretty safe, so far as our pres- 

 ent knowledge goes, in giving ourselves little concern about the num- 

 ber of bacteria which it may contain. 



