The Dictionary Of Photography

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

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Equivalence, Chemical
The principal bromides, chlorides, and iodides, which are likely to be used in emulsions of either gelatine or collodion, have been included in these tables. Table No. i presents to the reader, without any mystification which may be involved in equivalents, the actual weights of haloid or silver, as the case may be, required to convert or combine with one grain of the other. In order to test the utility of this table, let us suppose that it is desired to make (say) 10 ozs. of emulsion by a new formula, which, for the sake of showing the working of the table, we will write down as followsi - Bromide of potassium 150 grains. Chloride of ammonium 10 grains. Iodide of potassium 10 ,,           Gelatine ... ... 200 ..
Now, we want to know how much silver nitrate should be em-ployed in sensitising this mixture. For this purpose we use the first column, in which we find against each haloid the exact quantity of silver nitrate required to fully decompose one grain. Taking, then, the figures we find in column No. 1 against the three salts in the above formula, and multiplying them by the number of grains of each used, we have the following sum : -
or the total quantity of silver nitrate required for full conversion, 256 grains.
General Table of Formulce : Chemicals in frequent use. In giving the following table of formulae and molecular weights, all previously stated about the insufficiency of concise informa-tion, or even its misleading nature, must be emphasised. As examples, we may refer to the formula given for boracic or boric acid; the acid in this particular form and molecular value being rather a chemical abstraction - useful to be remembered by the student - than to be definitely weighed out; and the same remark replies to sulphurous acid. Again the inexperienced manipulator would have small or no chance of weighing out carbonate of potassium so as exactly to correspond to what he might gather from the formula K2C03; and alum might quite as reasonably have been represented by half the value assigned to it in our table. Again, in the case of ammonium carbonate, it was im-possible to give any formula corresponding with a commercial product. For the understanding of the table, it is necessary to