water, from which the gas is constantly tending to escape. The strength of the solution is therefore continually decreasing, thus leading to great uncertainty as regards the quantity actually present.
Ammonia of 22° Be' (density 0.923 at 6o° F.) contains 19 per cent of ammonia gas i.e. 39.5 per cent of the hypothetical hydrate English photographic formulae For use in quantity, caustic soda is best obtained in the form of lye, which is cheaper and always purer. Lye of 36° Be. (density, 1.332 at 6o° F.) contains 40 per cent of caustic soda (Na0H) ; the lye of 40 Be (density 1.383) contains 48 per cent. The use of one or other of these lycs allows of more correct dosage than the lump soda, which is always more or less hydrated.
Caustic Potash. Caustic potash has the same appearance and the same commercial forms as caustic soda, and its properties are closely similar. In handling it the same precautions as for caustic soda must be used. Potash lye of 36° Be. contains 44-5 per cent of caustic potash (KOH).
Caustic Lithia (LiOH). White powder,' which does not deliquesce but must, nevertheless, be kept from air to avoid carbonation. It is much less soluble in water than the alkalis proper (about 7 per cent at 6o° F.). With some developers, e.g. paramidophenol, it may be advantageously used (in much smaller quantity) in place of the caustic alkalis (A. and L. Lumiere, mmonia. From various points of view, ammonia may be compared with the caustic alkalis. It is a solution of ammonia gas in usually mention the saturated solution (density o-88o), containing about 35 per cent of gaseous ammonia, but much less stable than the one above. The loss of gas can be greatly reduced by diluting the solution, when purchased, with an equal quantity of water, twice the amount being taken for use.
Ammonia was the first alkali used with pyro prior to the introduction of gelatino-bromide emulsions, but being a solvent of silver bromide, it is apt to produce dichroic fog, so that its use has been discontinued except for a few special purposes.
Sodium Carbonate. Sodium carbonate is sup plied either as an anhydrous powder called Solvay salt, and having the appearance of flour, or as colourless crystals Jo Assuming both products to be pure, their equivalence is— part of anhydrous sodium carbonate 2-7 parts crystal carbonate.
part crystal sodium carbonate = 0-37 parts anhy drous carbonate.
The anhydrous carbonate tends to absorb atmospheric moisture, whereas the crystals effloresce ; both tend to change to a stable form, the monohydrate but the error from these variations is much less in the case of the anhydrous salt than in that of the crystals.' Both salts, but especially the crystals, absorb the carbonic acid from the air and thus partially change to bicarbonate.
The soda carbonate supplied as " washing " soda (" sal soda " in U.S.A.) for household use contains a large proportion of inactive sulphate and an excess of water ; it should not be used.
Sodium carbonate is very soluble in water. A saturated solution at F. contains more than 16 per cent of the anhydrous salt and 44 per cent of the crystals. At 86° F. these strengths are 36 per cent and 98 per cent respectively.
Potassium Carbonate. Potassium carbonate is supplied in the form of a white, powdered or granular, salt, which is highly deliquescent and must therefore be kept in a sealed container.
At 6o° F. a saturated solution contains 81 per cent of anhydrous potassium carbonate At the strengths usually employed in photo graphic developers the hydrolysis of the potassium carbonate (separation into caustic potash and bicarbonate) liberates a much greater amount of caustic alkali than does the hydrolysis of sodium carbonate, so that potassium carbonate gives developing solutions of greater energy. As its price is considerably higher than that of sodium carbonate, the latter is sometimes substituted, with the addition of about It) per cent of its weight of caustic soda.
Ammonium Carbonate. Ammonium carbon ate, used in making up some developers for warm-tone lantern slides, is supplied in the form of a salt, coming between neutral carbonate and bicarbonate, called ammonium sesquicarbonate ( 2 . 2 and forming white, waxy-looking lumps, smelling strongly of ammonia owing to their gradual conversion into bicarbonate. This salt is often covered with a white crust which must be rinsed off and the lumps rapidly dried before weighing. It is very soluble in cold water (about 20 per cent) and decomposes much below boiling point, so that its solutions must be prepared with cold water.