The Photographers' Book of Practical Formulae (Classic Reprint)

We are unluckily cursed with four different systems of weights - Avoirdupois, Troy, Apothecaries' and the Metric system - and these are often used indiscriminately by authors and those who compound formulae, and the for mulae generally given forth without stating which set of weights is to be taken as the standard. As a matter of course the metric system cannot be mistaken for any other; so it is with the first three that we have to deal. While in general practice, the avoirdupois, Troy and Apothecaries' weights are nearly enough alike to cause no very serious error in compounding our chemicals, still there can be no accurate results reached by those who are desirous of working by strictly correct weights, and who have sufficient chemical acumen to analyze their work.

So far as we know, the writers who specify what weight shall be employed can be counted on ten fingers The makers of dry plates are also at fault, and we can 'call to mind but one or two instances where a definite statement is made by them.

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