CHLORIDE OF ZINC. Zn. Cl. = 68, or, as produced by evapo rating its solution, Zn. Cl. H 0 = 77. It may be formed by evaporating a solution of zinc in hydrochloric acid to dryness, and heating the residue red-hot in a glass tube with a small aperture. The solution of this salt is always acid, and cannot be neutralized till all the zinc is precipitated. Zinc has a very powerful affinity for chlorine. Its acidity is not in favour of its use photographically : it is soluble in its own weight of alcohol.
collected over warm water, as cold water absorbs it. The qualities which distinguish chlorine in chemistry are its strong affinity for hydrogen and the metals, and its bleaching power : in photography, the increase of its usual affinities by the action of light. Its bleach ing power is a consequence of its affinity for hydrogen, since, on coming in contact with colouring matter in the. presence of moisture, it decomposes the water, especially in the light, and the nascent oxygen unites with the colouring matter to form a new and colour less compound. In the same way it destroys organic substances in the atmosphere, carrying with them infectious disorders, and is, therefore, used as a disinfectant : hydrogenous gases and miasmata are thus destroyed, as well as vegetable and animal dyes containing hydrogen, and even the colour of ink, which is formed by gallic acid, of which hydrogen is a component. Engravings which are produced with printers' ink, a mixture of charcoal, vegetable dyes, and organic oils and resins, are not proof against the action of chlorine : even the carbon in such combinations may, by the joint action of light and this gas, be carried away as chloro-carbonic acid. Photographs are particularly liable to be thus attacked. Many substances are decomposed by chlorine with such violence as to cause combustion. Many organic substances appear to be capable of forming with chlorine as many different compounds as they contain atoms of hydrogen, the hydrogen being displaced by chlorine by single atoms at a time, which go off as hydrochloric acid.
As has been said, the affinity of chlorine for hydrogen and carbon is much exalted by the sun's rays, and organic bodies, unaffected by chlorine in the dark, are often rapidly altered by it in the light, and sometimes the alteration takes place with explosive violence. The
nature of the effect produced on chlorine by light has not yet been determined : some suppose that it is rendered permanently allotropic ; others, that an effect is produced upon it similar to the induction of electricity on insulated conductors in the proximity of an electrified body, which state or effect immediately ceases on the withdrawal of the exciting force. But the investigations of more than one experi menter seem to prove, that an effect is produced upon chlorine in the light which does not terminate with the exposure to light, but con tinues in the dark, and the darkening which occurs in iodide of silver, exposed with nitrate to the sun, is only gradually removed when it is afterwards taken into a dark place. It has been said by some that the light combines with the element to form a definite compound, but this is not consistent with what is now generally admitted as to the nature of the phenomena of light, and is not accordant with the undulatory theory which is now considered b most philosophers to be established. The action on the elementary body is probably an induced polarity of some kind, which continues for a longer or shorter period, even after the disappearance of the exciting force.
Chlorine is the only body capable of dissolving gold and platinum, and is a more powerful body even than oxygen. It constitutes with oxygen, bromine, iodine, and fluorine, a natural group of very similar bodies, of which it is the most energetic. In testing for chlorine in the free state, it is necessary to disting-uish it from ozonized oxygen, which, being more active than ordinary oxygen, might, in some of its reactions, be mistaken for it.
At the temperature of 60° water dissolves twice its volume of chlorine, and more as the water is cooled : in this respect it resembles the binoxide of bydrogen, which gives off at high temperatures the oxygen which it holds in solution. Chlorine water acts like chlorine itself, but is decomposed by sunshine, with the formation of hydro chloric acid and liberation of oxygen.