160 ELEMENTS OF GENERAL SCIENCE 



not practicable. It would be extremely difficult to secure 

 from wells a supply sufficient for a large city, and, further- 

 more, the soil in cities becomes so filled with various waste 

 materials that the waters from surface wells would hardly 

 be suitable for use. The other available sources of supply 

 in most cases are lakes or rivers, both of which need much 

 study to make certain that they ars always safe. 



175. Importance of pure water. The importance of pure 

 water lies very largely in its relation to disease. We have 

 learned that many kinds of bacteria are commonly found in 

 water, and that no water is entirely free from them unless it 

 has been sterilized. If these bacteria are of the kinds that 

 produce disease, serious results may follow when the water 

 is used for drinking and for domestic purposes. 



The most important disease bacterium found in water is 

 the typhoid bacillus (Chap. IX). For that reason the death 

 rate from typhoid fever is often taken as a sort of index to 

 the condition of the water supply. There is given here a 

 graphic representation of the typhoid death rates in a num- 

 ber of cities (fig. 80). You will be able to draw your own 

 conclusions as to the safety of the different sources of supply. 



At one time Chicago had a typhoid death rate of 173 per 

 100,000 population, and at that time this was the highest 

 death rate from typhoid in the civilized world. The rate 

 was only 13.7 per 100,000 in 1910, 7.6 in 1912, and 5.1 in 

 1916. The changes in water supply and sewage disposal, 

 together with other modern methods of health control, have 

 been important factors in securing this reduction. 



176. Impurities in water. Some of the many kinds of im- 

 purities found in water are highly objectionable, while others 

 are of very little consequence. These substances may be 

 grouped in three classes: dissolved substances, sediments, 

 and bacteria. 



Certain dissolved substances in water produce what is com- 

 monly known as hard water. Hard water is an annoyance 



