The impurities of commercial Iodide of Potassium, with the means to be adopted for their removal, are fully given at page 197. IODIDE OF SILVER. _See_ Silver, Iodide of. IODOFORM. The composition of this substance is analogous to that of Chloroform, Iodine being substituted for Chlorine. It is obtained on boiling together Iodine, Carbonate of Potash, and Alcohol. Iodoform occurs in yellow nacrous crystals, which have a saffron-like odour. It is insoluble in water, but soluble in spirit. IRON, PROTOSULPHATE OF. Symbol, FeO SO{3} + 7 HO. Atomic weight, 139. The properties of this salt, and of the two salifiable Oxides of Iron, are described at page 29. It dissolves in rather more than an equal weight of cold water, or in less of boiling water. Aqueous solution of Sulphate of Iron absorbs the Binoxide of Nitrogen, acquiring a deep olive-brown colour: as this gaseous Binoxide is itself a reducing agent, the liquid so formed has been proposed as a more energetic developer than the Sulphate of Iron alone (?). IRON, PROTONITRATE OF. Symbol, FeO NO{3} + 7 HO. Atomic weight, 153. This salt, by careful evaporation _in vacuo_ over Sulphuric Acid, forms transparent crystals, of a light green colour, and containing 7 atoms of water, like the Protosulphate. It is exceedingly unstable, and soon becomes red from decomposition, unless preserved from contact with air. The preparation of solution of Protonitrate of Iron for developing Collodion Positives, is given at page 206. IRON, PERCHLORIDE OF. Symbol, Fe{2}Cl{3}. Atomic weight, 164. There are two Chlorides of Iron, corresponding in composition to the Protoxide and the Sesquioxide respectively. The Protochloride is very soluble in water, forming a green solution, which precipitates a dirty white Protoxide on the addition of an alkali. The Perchloride, on the other hand, is dark brown, and gives a foxy-red precipitate with alkalies. _Properties._--Perchloride of Iron may be obtained in the solid form by heating Iron wire in excess of Chlorine; it condenses in the shape of brilliant and iridescent brown crystals, which are volatile, and dissolve in water, the solution being acid to test-paper. It is also soluble in Alcohol, forming the Tinctura Ferri Sesquichloridi of the Pharmacopœia. Commercial Perchloride of Iron ordinarily contains an excess of Hydrochloric Acid. LITMUS. Litmus is a vegetable substance prepared from various _lichens_, which are principally collected on rocks adjoining the sea. The colouring matter is extracted by a peculiar process, and afterwards made up into a paste with chalk, plaster of Paris, etc.