The Dictionary Of Photography

A True Historic Record Of The Art & Practice Of Photography 100 Years Ago.

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Weighing and Measuring
liquid the photographer has to measure is hot water - are most conveniently measured in thin German glass beakers, which can be obtained from the dealers in chemical apparatus, and which will seldom break when hot water is poured in. Graduation by marking on the outside with a writing or scratching diamond is not always satisfactory, as the scratches are often starting points for fracture ; hence it is generally better to gum on a strip of paper, to mark the graduations on this, and finally to varnish. In thus graduating a measure it is convenient to weigh in successive quantities of water, and a mark is made on the paper strip at each addition. The beaker being counterpoised on the scale pan, the required weights are put into the other pan (a fluid ounce of water is the volume occupied by an ounce avoirdupois), and equilibrium is brought about by pouring water in the beaker; the final adjustment being most conveniently made with a pointed syringe. The usual minim measure is not convenient for measuring very small quantities, and estimation of such small quantities by counting drops is a tedious strain on the observing powers, moreover, very inaccurate, as drops even of the same liquid vary very much in size. For example, a drachm of water may give from 31 to 54 drops, according to the shape of the vessel from which it is allowed to drop. Mr. Alsop's minim-meter (fig. 135) is one of the most convenient arrangements. It is virtually a small syringe, and in using it the piston is raised half an inch or so, and the point of the syringe is then dipped into the liquid and the required quantity is drawn up, or a little more may be drawn up; and any excess, beyond the quantity required, is forced out by carefully depressing the piston, after which the measured quantity is expelled. A certain amount of air should always intervene between the piston and the liquid to be measured. The measure is readily cleaned by separating the parts and washing them separately. A length of glass tube of suitable size and 1ยง" I35' tolerably uniform in the bore will make a number of
these minim measures (see Glass Working); and instead of