CHLORIDE. The' compounds of other elements with chlorine are either acid.s or chlmides. With oxygen and hydrogen it forms acids, vrith metals and other elements chlorides. The chlorides of the alkalis and alkaline earths are used in photography not as sensitive bodies, but as convenient salts for the production of those which are. The chlorides which would be especially likely to be affected by the light falling upon them, are the compounds 'of silver, platinum, and metals not easily oxidized ; and also such metals as iron, which have several oxides and chlorides, and combine in more complex forms than those whose affinities for oxygen and chlorine are much stronger, and whose salts are much simpler. Iron, for instance, has four oxides and two chlorides, and the oxygen and chlorine appear to have the power of replacing each other in some cases. Their relations' resemble those of organic compounds more nearly than those of many simpler inorganic bodies, and would therefore appear to promise the production of compounds suitable for the production of photographic images. Cobalt, copper, iron, and manganese are especially remarkable for these properties, and many have been already found to be decomposed, or modified, by light and organic bodies. Chlorine appears to have a greater affinity for most of the metals than oxygen, so that it is removed by heat and light with greater difficulty ; and though chlorine at a high temperature decomposes most of the oxides, and entirely disengages the oxygen, oxygen acts only on a very few chlorides, principally of the class which has been spoken of as forming complex arrange ments. Iron seems to be nearly balanced between chlorine and oxygen, for sesquioxide of iron dissolves in hydrochloric acid to form sesquichloride of iron and water; but the sesquichloride, evaporated and exposed to the action of the air, again forms sesquioxide and hydrochloric acid. There can scarcely be a doubt that this last action would be accelerated by light. Water is decomposed at ordinary
temperatures by the perchlorides of manganese, chromium, colum bium, and tungsten, and by the chloride of arsenic : hydrochloric acid and a metallic acid are the results. Chloride of tungsten is decomposed by it, with the disengagement of hydrochloric acid, and the precipitate of oxide of tungsten of a violet color. Water also acts upon the chlorides of antimony, bismuth, and tellurium, decom posing them into oxychlorides which are insoluble, and chlorides soluble in the atoms of hydrochloric acid formed. In the reaction of water upon the subchloride of tellurium some of the metal is even reduced to the metallic state. The chlorides of silver, platinum, gold, 8m., are believed to be all decomposable by water and light, more or less easily. Some that do not appear to form hydrochloric acid with water, as the bichloride of mercury, do so in the light in ether, or other solvents. The alkaline and earthy chlorides, whose con- . netting affinity is stronger, do not appear to be oxidized or reduced by water, in which they are very soluble; but chloride of sodium and others will speedily give colour in the light to alcohol and other hydrogenous solvents. It is impossible not to see in all this that the affinity which is instrumental in causing these changes is that of hydrogen for chlorine, nor can the mind escape the impression that this is all extremely interesting as an illustration of the probable action of light in most of the instances in which its chemical effects have been observed. Many of these reactions of water and chlorides have been observed by chemists, without any attention to the share which light might take in the operation : but this is a subject highly worthy of investigation. As the chlorides are most of them used to form chloride of silver, their equivalents and solubility in water, or insolubility, should be attended to.