Foods: their composition and analysis : A manual for the use of analytical chemists and others. With an introductory essay on the history of adulteration / By Alexander Wynter Blyth. With numerous tables and illustrations.

  • Alexander Wynter Blyth
Date:
1882
Catalogue details

Licence: Public Domain Mark

Credit: Foods: their composition and analysis : A manual for the use of analytical chemists and others. With an introductory essay on the history of adulteration / By Alexander Wynter Blyth. With numerous tables and illustrations. Source: Wellcome Collection.

Provider: This material has been provided by The University of Leeds Library. The original may be consulted at The University of Leeds Library.

    559/673 (page 514)
    to be pi-ovicled Avith a colourless glass tube, at least 2 feet in. length, having the ends closed with plate glass, and a small opening in the side of the tvibe through which to pour the water. A cheaper method of securing an aperture through which to introduce the water is to have a segment cut out of one of the glass discs, or a segmental section out of the end of the tube itself; the most convenient diameter of the tube is 2 inches, but one greater or smaller will answer the purpose. To make an observation, the tube is half-filled with the water to be examined, and then directed towards a white surface, which may be a white cloud in the sky or an equally illuminated sheet of paper. The air-filled space above the water then affords an excellent semi- circular disc of comparison, and renders it easy to detect the slightest shade of colour. The purest watei-s have the slightest tinge of blue ; the next in order of purity have a just distinguish- able shade of green. Decided green tints, London fog hues, amber yellow, and brown tints ai'e those possessed by waters tinged with peat, containing suspended matters, of second class composi- tion, or those of considerable impurity. * Smell.—Half a litre of the water or more is warmed in a large corked or stoppei-ed flask to 38° [100° F.] ; a long glass tube of three-quarters of an inch in diameter is now inserted, and the water sucked up once or twice so as to wet the side of the tube thoroughly] then, without taking the tube out of the flask, one nostril is applied to the orifice of the tube, the other closed by the finger, and deep inspirations or sniff's taken. Another simpler plan is to warm a quantity of the water, with- out removing the stopper, up to the temperature given, then shake, remove the stopper, and smell; a putrid odour denotes decomposing animal or vegetable matter. If the sample is much polluted by fresh sewage, a urinous odour is not unfrequently distinct. But, again, it may be specially noted that water quite unfit to di'ink may have no odour, hence the usefulness of the test is limited. A posi- tive smell teaches volumes—a negative result is of little value. Taste.—A few waters, and a few only, have a decided taste. It is scarcely to be recommended that analysts should taste samples derived from fever-stricken localities ; but, on the other hand, when there is no suspicion of the samples having been the * Messrs. Crookes, Odling, and Tidy, in tlicir report on the London waters supply for ISSl, describe an ingenious colour meter,'' consisting of two hollow wedges tilled, one with a' brown and the other with a blue solution. Any desired combination of green and blue may be made by sliding the wedges across each other in front of a circular aperture in a sheet of metal, and thus imitating the tint of water under examination ; each prism is graduated from 1 to 50, the figures representing millimetres of the thickness at tliat particular part of the prism.