Heated in a crucible the salt melts, and when poured into a mould and solidified, forms the _lunar caustic_ of commerce. At a still higher temperature it is decomposed, and bubbles of oxygen gas are evolved. The melted mass, cooled and dissolved in water, leaves behind a black powder, and yields a solution which is faintly alkaline to test-paper. The alkalinity depends upon the presence of _nitrite_ of silver associated with excess of oxide, in the form probably of a basic or _sub_-nitrite of silver.[B] [Footnote B: Nitrite of silver differs from the nitrate in containing less oxygen, and is formed from it by the abstraction of two atoms of that element.] Solution of nitrate of silver is decomposed by iron, zinc, copper, mercury, etc., the nitric acid and oxygen passing to the other metal, and metallic silver being precipitated. Silver, Nitrite of. Symbol, AgO NO{3}. Atomic weight, 154. Nitrite of silver is a compound of nitrous acid, or NO{3}, with oxide of silver. It is formed by heating nitrate of silver, so as to drive off a portion of its oxygen, or more conveniently, by mixing nitrate of silver and nitrate of potash in equal parts, fusing strongly, and dissolving in a small quantity of boiling water; on cooling, the nitrite crystallizes out, and may be purified by pressing in blotting paper. Mr. Hadow describes an economical method of preparing nitrite of silver in quantity, viz. by heating 1 part of starch in 8 of nitric acid of 1·25 specific gravity, and conducting the evolved gases into a solution of pure carbonate of soda until effervescence has ceased. The nitrite of soda thus formed is afterwards added to nitrate of silver in the usual way. _Properties._--Nitrite of silver is soluble in 120 parts of cold water; easily soluble in boiling water, and crystallizes, on cooling, in long slender needles. It has a certain degree of affinity for oxygen, and tends to pass into the condition of nitrate of silver; but it is probable that its photographic properties depend more upon a decomposition of the salt and liberation of nitrous acid. _Properties of Nitrous Acid._--This substance possesses very feeble acid properties, its salts being decomposed even by acetic acid. It is an unstable body, and splits up, in contact with water, into binoxide of nitrogen and nitric acid. The peroxide of nitrogen, NO{4}, is also decomposed by water and yields the same products. Silver, Acetate of. Symbol, AgO (C{4}H{3}O{3}). Atomic weight, 167. This is a difficultly soluble salt, deposited in lamellar crystals when an acetate is added to a strong solution of nitrate of silver. If _acetic acid_ be used in place of an acetate, the acetate of silver does not fall so readily, since the nitric acid which would then be liberated impedes the decomposition. Silver, Hyposulphite of. Symbol, AgO S{2}O{3} . Atomic weight, 164.