581 
for the sake of illustrating the variations which may occur in the 
results of a bacteriological examination of water. 
Wliat has just been said in regard to the chemical and bacterio- 
logical examination of water does not apply of course to the applica- 
, tion of chemical and bacteriological examinations in general. On 
the contrary, it is possible to detect specific causes of disease by 
these means in many cases. One need only recall the value of the 
chemical and bacteriological examinations of urine in diseases of the 
kidney and bladder, and of the value of bacteriological examina- 
tions in cases of suspected tuberculosis and diphtheria. But on the 
other hand while such examinations of water supplies may be of 
very great value, they give us only the probabilities in the case, 
and these probabilities are open to some difference of opinion as to 
their weight. The more experience the observer has had the less 
inclined he is to make arbitrary standards and the more capable is 
he of forming a correct judgment in his interpretation of the results 
of Ins examination. In the examples given above of the detection 
of Iddney disease and of the specific bacteria in diphtheria and 
tuberculosis, competent observers will all readily agree in the inter- 
pretation of results. In the matter of the probability of pollution 
of water there is more room for difference of opinion from results 
of analysis. The various statements made in text-books and in 
monographs on hygienic water analysis give ample evidence of the 
want of uniformit}" of opinion in this respect. It would seem essen- 
tial in all cases to establish a standard of purity for the region of 
country from which any specimen of water under consideration 
comes. 
There are competent observers with abundant experience who are 
inclined to question the value of chemical and bacteriological water 
analysis in toto, and in view of the arbitrary and mechanical manner 
in which the results of these analyses are sometimes interpreted this 
attitude is justified to some extent. It would seem, however, that 
after the establishment of normal standards for a given locality such 
analyses are useful if they are checked by intelligent consideration 
of all the conditions entering into the case but no hard and fast 
rules can be applied. 
PURIFICATION OP WATER IN THE SOIL. 
While water in its passage through the air and through the soil 
becomes contaminated with bacteria which may cause special disease 
or disturbance of digestion, it also undergoes on the other hand a 
process of purification, consisting in a filtration of the particles held 
in suspension. It has been found that at a comparatively short dis- 
tance below the surface, 4 or 5 feet, there are frequently but few bac- 
