CHEMICAL EXAMINATION OF THE WATEK SUPPLY. 279 
dissolved oxygen, and oxygen consumed, together with the physical 
characteristics of the water, such as color, odor, turbidity, and sedi- 
ment. In the time at our disposal, and for other reasons, it has not 
been found possible in all cases to determine the color and turbidity 
in terms of the standards now in use for these purposes. However, 
in all cases the best general description possible has been given of the 
several samples of water analyzed, and in all cases in which it has 
been found possible to do so comparison has been made with the 
fixed standards for color and turbidity, and in our tables the num- 
bers obtained have been placed alongside of the general description 
of the water. The waters of the shallow wells were found to be so 
clear and colorless as to render these determinations unnecessarv, 
and on account of the iron which they contained the waters of the 
deep wells altered so much in color and turbidity on standing after 
the sample had been collected that such determinations are without 
significance. Except in the determination of total solids, the sev- 
eral determinations on all samples of water were begun as soon as the 
sample was brought into the laboratory and completed the same day 
as collected, so that at most only a few hours elapsed between the 
collection of the sample and the completion of the chemical examina- 
tion thereof. The determination of total solids, includiug the total 
residue on evaporation, the mineral residue, and the volatile matter, 
was in most instances also begun within a few hours after the collec- 
tion of the sample, and whenever for an}^ reason evaporation had to 
be postponed this determination has been made at another date on a 
duplicate sample of the water, and in the few instances in which we have 
had reason to suspect the solution of the glass of the container by the 
water of the sample a note has been made to this effect. On account 
of the great humidity of the atmosphere of the laboratory during the 
month of August — a month of excessive rainfall in the District of 
Columbia — and the extremely deliquescent character of certain of 
the total residues of the shallow wells, we were compelled to postpone 
the determination of total solids of certain of these waters until Sep- 
tember or October, when determinations were made on duplicate 
samples of the waters. Finally, it should be noted in passing that 
whenever possible our standards and solutions were checked by com- 
parative determinations on solutions of chemically pure salts. 
NUMBER OF SAMPLES OF WATER ANALYZED. 
Since July 16, 1906, on which date the bacteriological and chemical 
examinations of the water supply of the District were begun, sanitary 
analyses of 283 samples of water have been made in the chemical 
division of the Hygienic Laboratory. These have included: 
(1) Fourteen complete sets of analyses of waters of the reservoirs, 
the samples of which were taken at weekly or semiweekly intervals. 
