The Dictionary Of Photography

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

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Equivalence, Chemical
ence by reason of excess of either substance, we can take the quantities indicated by the equation, these being as follows: -
K Br + Ag N 03 = Ag Br + K Nt03. 39 80 108 14 16 108 80 39 14 16
_3                        * _3
48                                 48
119
170
101
Although tabular matter and lists of chemical formulae may mislead one unversed in chemical matters, and will be quite superfluous to one skilled in chemistry, certain tables are sub-joined, the first being a table of Dr. Eder's, which will be of service to emulsion makers, as it shows in plain figures the actual weight of various salts required to react with 170 parts by weight of silver nitrate. It will be noticed that the fractional parts involve a slight departure from the figures which would be obtained from our table of atomic weights, a matter sufficiently explained by our previous remarks. As a guide to practice, even this very clear and explicit table requires special knowledge, for many reasons. In the first place, it is usual in making emulsions to employ a great excess of the compound required to decompose the silver nitrate, and the question of water of crystallisation, shown in the formulae as H20(== 18), 2H20 (= 36), 4H20 (= 72), etc., steps in as a disturbing factor very often, the salts having lost water or gained water from the atmosphere. (See Efflo-rescence ; also Deliquescence.
Dr. Eder's table of equivalents of Haloid Salts which are required to decompose 170 parts of sllver Nitrate.
58-5
Sodium chloride ...
.. (NaCl).
74-6
Potassium chloride
.. (KC1).
53'5
Ammonium chloride
.. (NH4C1).
55'5
Calcium chloride ... ... ■
.. (CaCLJ.
1095
Crystal calcium chloride ...
.. (CaCL6H20).
133*2
Strontium chloride
.. (SrCL6H20).
79-2
Anhydrous strontium chloride
.. (SrCl2).
42*5
Lithium chloride ...
.. (LiCl).
101-5
Magnesium chloride
.. (MgCl26H20)
47-5
Anhydrous magnesium chloride 282
.. (MgCl2).