Science of hygiene : a text-book of laboratory practice for public heatlh students / by Walter C.C. Pakes.

  • Pakes, Walter C. C.
Date:
[1912]
Catalogue details

Licence: In copyright

Credit: Science of hygiene : a text-book of laboratory practice for public heatlh students / by Walter C.C. Pakes. Source: Wellcome Collection.

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    grains per gallon, it is necessary only to multiply by 07. The reasons for this, of course, are that there are 100,000 grammes in 100,000 c.c. and 70,000 grains in a gallon. PHYSICAL CHARACTERS Before proceeding to the analysis of the water by the various chemical methods which are adopted, it is generally necessary to make some preliminary observations on the physical characters of the sample. 1. Turbidity. Waters vary somewhat in the amount of turbidity they show, although as a rule the best waters are per- fectly clear. It is of course possible that a perfectly safe drinking water might have been rendered turbid through the shaking up with it of some mineral matter, but, speaking generally, a water that shows any marked turbidity will prove, upon further examination, to be unfit for drinking purposes. One expresses the degrees of turbidity as “ clear,” “ slightly turbid,” or “very turbid,” the last applying more especially to sewage or sewage effluents. 2. The next point to be considered is the colour. In order to determine the colour it is necessary first of all to allow any sediment to deposit. The clear supernatant water is then poured into what is known as a two-foot tube. This consists of a glass cylinder, at each end of which a piece of plate glass is screwed. When the water is poured in, the plate which has been removed in order to allow of this should be screwed down, and the whole apparatus carefully wiped. The eye should then be placed at one end, and at the other a white porcelain slip or piece of paper. Good waters are generally slightly blue when seen through this tube; if there is any yellowish or brownish colour, there will be some suspicion of sewage contamination unless the water happens to have been collected from a peaty soil. Occasionally the water possesses a decidedly green colour owing to the presence of the green algae, which are of course, in themselves, harmless. 3. Taste. This is by no means a valuable test; the pleasant taste of a water is generally due to the solution of gases, and even the best of waters when not aerated are insipid. When it is remembered that a smaller quantity of sodium chloride than 75 grains to the gallon or 100 grammes to the 100,000 cannot be
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