CHEMICAL VS BACTERIAL EXAMINATION 271 
Sawyer (1915) quotes an instance of a water-borne epidemie which could 
have been prevented had it been possible to carly out a sanitary inspec- 
tion. In a certain water supply indications of pollution appeared two 
months before the epidemic broke out. Could a sanitary inspection 
have been made to investigate the conditions indicated in the laboratory 
results, seven deaths and a large number of cases of typhoid fever could 
have been prevented. In many cases it would not be necessary to make 
laboratory examinations because the environmental factors would 
indicate a bad water. In such a case, the laboratory might be used to 
check up the remedial measures which had been carried out. The 
field investigations would also yield information with regard to the 
endemic disease which occurs in a community. Where possible, both 
field studies and laboratory studies should be made but where one 
is possible, the sanitary inspection will make it unnecessary, very often 
to carry out the laboratory examination. 
Pure Water. Chemically water is a monoxide of hydrogen and is 
made chemically pure only with great difficulty. Only the greatest 
care in technique will yield a water which is free from all traces of 
metals and gases. 
Bacteriologically speaking, a water is pure when it contains no bac- 
teria which may cause disease or sickness. This is a difficult term to 
define since a water which is pure for one individual may be impure for 
another. The technique which is available for isolating certain path- 
ogenic bacteria is too crude to give accurate knowledge with regard to 
purity. Negative results may mean little more than that the certain 
bacteria sought for, were not found. Generally speaking, a pure water 
is one which will not cause any disturbance in those who partake of it. 
Relation of Chemical and Bacterial Exan ination. In the past there 
has been much discussion over the superiority of the chemical or bac- 
terial analysis of water. This has probably been quite useless since 
both are important and necessary for an accurate opinion with regard 
to the potability of a given water. The more data which an analyst 
has, the more accurate will his opinion be. Despite all this there are 
certain outstanding facts with regard to each analysis which might be 
emphasized. 
The chemical analysis may give more data with regard to the history 
and possibly the age of the water. It gives an idea with regard to the 
organic matter and the different stages in its decomposition. If most 
of the nitrogen is in the form of nitrates, it indicates possibly remote 
pollution and a partial purification. In giving an opinion which must be 
based on the chemical analysis only, the water analyst has to use the 
