The presence of these organisms, however startling some of them may be in appearance, is usually not objectionable; indeed, microscopic vegetable growths are frequently of service in the purification of potable water. The more important forms of bacteria (bacilli, etc.), present minute rod-like shapes, far less impressive in appearance.
Considerable difference of opinion exists in regard to the sanitary value of the results afforded by the biological examination of water. While the number of bacteria found in a given quantity of water may be of aid in the formation of an opinion as to its relative safety for domestic purposes, it should be borne in mind that these micro-organisms are almost omnipresent, being contained in the air, and in soils, and articles of food.
The following tabulation shows the relative purity of the water supply of several American cities, as determined by Prof. A. R. Leeds, in June, 1881:—
| Parts in 100,000 | New York. | Brooklyn. | Jersey City. | Philadelphia. | Boston. | Washington. | Rochester. | Cincinnati. |
| Free ammonia. | 0·0027 | 0·00075 | 0·00475 | 0·001 | 0·01325 | 0·006 | 0·0114 | 0·0115 |
| Albuminoid ammonia. | 0·027 | 0·00825 | 0·0427 | 0·018 | 0·0605 | 0·027 | 0·023 | 0·024 |
| Oxygen required. | 0·81 | 0·413 | 0·95 | 0·46 | 1·77 | 0·600 | 0·790 | 0·860 |
| Nitrites. | None | None | None | None | None | None | None | None |
| Nitrates. | 0·8325 | 1·2025 | 0·9065 | 0·6845 | 1·2395 | 0·8325 | 0·629 | 0·740 |
| Chlorine. | 0·350 | 0·550 | 0·235 | 0·3000 | 0·315 | 0·270 | 0·195 | 0·805 |
| Total hardness. | 3·300 | 2·270 | 3·200 | 4·400 | 2·100 | 4·800 | 5·500 | 6·400 |
| Total solids. | 11·800 | 6·000 | 9·300 | 14·300 | 8·500 | 11·500 | 10·000 | 16·200 |
| Mineral matter. | 5·000 | 5·000 | 3·400 | 6·000 | 2·000 | 5·500 | 4·000 | 9·000 |
| Organic and volatile matter. | 6·800 | 1·000 | 5·900 | 8·300 | 6·500 | 6·000 | 6·000 | 7·200 |
| Parts per 100,000. | April. | May. | June. | July. | August. | September. | October. | Nov. | Dec. | ||||||
| 3rd. | 6th. | 26th. | 13th. | 30th. | 15th. | 30th. | 19th. | 31st. | 15th. | 29th. | 14th. | 30th. | 15th. | 1st. | |
| Appearance, &c. | Cl. | Sl. Tb. | Tb. | Cl. | Cl. | Cl. | Cl. | Cl. | Cl. | Cl. | Cl. | Cl. | Cl. | Cl. | Cl. |
| Odour (heated to 100° Fahr.). | None | None | None | None | None | None | None | None | None | None | None | None | None | None | None |
| Chlorine in chlorides | 0·278 | 0·348 | 0·244 | 0·348 | 0·226 | 0·279 | 0·226 | 0·278 | 0·313 | 0·209 | 0·208 | 0·272 | 0·243 | 0·312 | 0·295 |
| Equivalent to sodium chloride. | 0·459 | 0·575 | 0·400 | 0·574 | 0·374 | 0·459 | 0·374 | 0·459 | 0·517 | 0·344 | 0·343 | 0·458 | 0·400 | 0·515 | 0·486 |
| Phosphates | None | None | None | None | None | None | None | None | None | None | None | None | None | None | None |
| Nitrites | None | None | None | None | None | None | None | None | None | None | None | None | None | None | None |
| Nitrogen in nitrates and nitrites. | 0·0403 | 0·0494 | 0·034 | 0·0469 | 0·0371 | 0·0395 | 0·0387 | 0·0469 | 0·0486 | 0·037 | 0·0477 | 0·041 | 0·047 | 0·048 | .. |
| Free ammonia | 0·001 | 0 | 0·002 | 0·003 | 0·0005 | 0·002 | 0·003 | 0·003 | 0·001 | 0·004 | 0·002 | 0 | 0·003 | 0·001 | 0·0032 |
| Albuminoid ammonia | 0·009 | 0·0166 | 0·0086 | 0·007 | 0·014 | 0·008 | 0·011 | 0·014 | 0·009 | 0·016 | 0·0094 | 0·013 | 0·015 | 0·014 | .. |
| “Hardness” equivalent to carbonate of lime— | |||||||||||||||
| Before boiling | 4·73 | 4·082 | 4·280 | 3·860 | 4·968 | 4·268 | 4·332 | 4·586 | 4·332 | 5·096 | 3·949 | 4·520 | 4·512 | 3·840 | 3·729 |
| After boiling | 4·31 | 3·787 | 3·510 | 3·500 | 4·586 | 4·268 | 4·332 | 3·949 | 4·332 | 4·459 | 3·822 | 4·294 | 4·512 | 3·390 | 3·164 |
| Organic and volatile (loss on ignition). | 6·00 | 1·500 | 3·00 | 2·50 | 2·00 | 0·50 | 2·50 | 3·00 | 2·00 | 2·50 | 2·50 | 2·50 | 3·00 | 3·00 | .. |
| Mineral matter (non volatile). | 5·00 | 4·000 | 4·50 | 4·50 | 5·50 | 5·00 | 5·00 | 5·00 | 4·50 | 4·00 | 4·00 | 4·00 | 4·50 | 4·00 | .. |
| Total solids (by evaporation). | 11·00 | 5·50 | 7·50 | 7·00 | 7·50 | 5·50 | 7·50 | 8·00 | 6·50 | 6·50 | 6·50 | 6·50 | 7·50 | 7·00 | .. |
Cl. signifies clear. Sl. Tb., slightly turbid. Tb., turbidity somewhat more marked.
Parts per 100,000.
| Description of Sample. | Date when taken. | Time when drawn. | Appearance in two-foot Tube. | Odour when heated to 38°. | Chlorine in Chlorides. | Equivalent to Sodium Chloride. |
| 1884. | ||||||
| Mohawk River, above Diamond Woollen Mills | Dec. 5 | .. | Turbid, greenish yellow | Faint aromatic | 0·233 | 0·371 |
| Hudson River, above Lansingburg | Nov. 12 | .. | Faintly turbid, light greenish yellow | Faint, vegetable | 0·233 | 0·371 |
| Troy hydrant | „ 12 | .. | Faintly turbid, light greenish yellow | Faint, vegetable | 0·233 | 0·371 |
| Hudson River, at Maple Island | Dec. 6 | .. | Faintly turbid, greenish yellow | Faint, marshy | 0·167 | 0·265 |
| Hudson River, at inlet | Nov. 1 | High tide | Clear, light yellow | Faint | 0·333 | 0·530 |
| Hudson River, at inlet | „ 1 | „ | Faintly turbid, light yellow | Faint, stale | 0·333 | 0·530 |
| Hudson River, at inlet | „ 1 | Low tide | Clear, brownish yellow | Faint | 0·366 | 0·583 |
| Hudson River, at inlet | „ 1 | „ | Turbid, brownish yellow | Faint | 0·366 | 0·583 |
| Hudson River, 50 ft. south of inlet | Dec. 4 | High tide | Slightly turbid, brownish yellow | Oily | 0·233 | 0·371 |
| Hudson River, at inlet | „ 4 | Low tide | Turbid, brownish yellow | Oily | 0·183 | 0·291 |
| Bleecker Reservoir | Nov. 1 | .. | Clear, faint yellow | .. | 0·340 | 0·541 |
| Bleecker Reservoir | „ 1 | .. | Turbid, faint yellow | Faint, stale | 0·340 | 0·541 |
| Tivoli Lake | „ 6 | .. | Faint milkiness | Faint, stale | 0·833 | 1·325 |
| Tivoli Lake | „ 6 | .. | Whitish, milky | Faint, stale | 0·833 | 1·325 |
| Tivoli Lake | Dec. 4 | .. | Turbid, greenish | Faint, marshy | 0·966 | 1·537 |