THE EXAMINATION OE WATER 653 



To isolate the organism from the concentrated deposit : — 



Plate in Conradi-Drigalski agar or litmus lactose bile-salt agar. 



Subculture the colonies into broth and work out their attributes in 

 other media. 



Try agglutinating (Widal) reactions in each subculture. 



Do a Pfeiffer's test, depending upon bacteriolysis to clinch a 

 diagnosis. 



(/) To detect tlic cholera vibrio. (See " Cholera.") 



(5) THE CHEMICAL EXAMINATION OF WATER (after Stewart). 



The results of water anahses are expressed in parts per 100,000 or 

 as grains per gallon = parts per 70,000, since there are 70,000 gr. per 

 gallon. 100 c.c. is a convenient quantity to work with for certain 

 estimates. When one is using standard solutions of a strength 

 I c.c. ^= o'ooi grm., the number of c.c. used in the titration gives the 

 result in parts per 100,000. Likewise, if 70 c.c. are taken of the 

 sample, the actual titration gives the result in grains per gallon. This 

 much facilitates the work as is demonstrated in a simple example 

 thus : — 



In estimating chlorine in a water sample, 3*4 c.c. of standard silver 

 nitrate solution (i c.c. = o'ooi grm. of chlorine) were required to reduce 

 it. Hence there were 3*4 parts of chlorine in 100,000 parts of the water 

 because : — 



3"4 X o-ooi X 100,000 



=3"4 



100 



To convert grains per gallon to parts per 100,000 multiply the 

 result by 10 and divide by 7, and conversely to multiply bv 7 and divide 

 by 10 gives grains per gallon. 



One is required to ascertain the 



(1) Total solids (6) Nitrates 



(2) The degree of hardness (7) Oxidizable organic matter 



(3) The metals present (8) Saline ammonia 



(4) Chlorine (9) Albuminoid ammonia 



(5) Nitrites (10) Degree of aeration 



in the ordinary drinking-water and sewage. 



These will be dealt with seriatim, applied to drinking-water first. 



(1) To Estimate the Total Solids. 



Weigh a small porcelain dish. 

 Take 200 c.c. of the water sample. 



Evaporate to dryness in dish over a water bath, pouring water in 

 as it evaporates until 200 c.c. have been poured in. 

 Dry the dish in a water oven for an hour. 

 Cool, weigh the dish and residue. 



