Page 162 - The Wellington photographic handbook

THE general custom in formulae is to use the word ounce to
imply the apothecaries' ounce of 480 grains, but if the
worker seeks to buy an ounce of any chemical from his
chemist he will be served with the avoirdupois ounce of 437
£
grains. Hence arises a little inconvenience, since an ounce avoir-
dupois as bought is insufficient to make up a formula in which
an apothecaries' ounce is prescribed. Fortunately for the photo-
grapher it is only rarely that the difference of approximately ten
per cent, between the two " ounces " is likely to appreciably affect
his results. In this book, whenever absolute accuracy is desirable,
grains are specified instead of fractions of an ounce. In the case
of pyro, which is usually sold in ounce bottles, it will be sufficient
if the whole ounce is dissolved as bought, and most formulas are
based on the assumption that this will be done.
A most convenient way of measuring small quantities of
chemicals such as potassium bromides is by means of percentage
solutions. It will be sufficient for all practical purposes to consider
a one, five or ten per cent, solution as one in which one, five or ten
units by weight of a salt are dissolved in water (or other liquid) to
make 100 units by volume of solution. For example, a ten per
cent, solution is one in which 1 ounce (480 grains) is dissolved
in water to make 10 ounces (4,800 minims) of solution. Then
every ounce of this solution will contain 48 grains of the
salt, every dram 6 grains, every 10 minims 1 grain
and every minim (or drop) 1 grain.
y^
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