Page:Popular Science Monthly Volume 11.djvu/651

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their limit of elasticity, whereby the molecules, brought to a state of flow, glide over each other, and assume a wholly new position more favorable to resistance, that causes the increase of elasticity. A simple condensation produces merely an increase of the absolute solidity and diminution of the tenacity, but no real increase of elasticity. The limit of elasticity may be raised nearly to the breaking consistence, so that, in many cases, it is six and seven times the original. Mere stretching for a short time is of little use; the tension must last a considerable time. It is also well to apply a gradually-increasing weight.

Properties of White Paint.—As the result of hundreds of experiments in carbonate of lead and hydrate of lead, both separately and mixed, Messrs. Wigner and Harland, of the British Society of Public Analysts, reach the conclusion that a white paint, to be efficient, and to possess both the powers of laying on readily and easily, and by its opacity hiding the color beneath, must consist of an admixture of hydrate and carbonate of lead in a certain definite proportion. The true proportion would appear to be three equivalents of carbonate of lead and one equivalent of hydrate. The experiments further show why zinc-white, carbonate of magnesia, and other metallic oxides and carbonates, do not yield good paints. In the case of white-lead a positive chemical compound has been formed, and the seventy-five per cent, or thereabouts, of carbonate of lead present has been dissolved in the chemical compound, and so a paint has been formed which possesses a covering power in excess of any other compound known. Until some means can be devised by which oxide of zinc or some other substance can be dissolved in the same way, so as to form a paint possessing characters somewhat different from a mere emulsion, it is vain to expect that they can equal good white-lead.

Change of Tint in Flowers.—The change of tint in flowers of Ipomoa purpurea (morning-glory) under the influence of atmospheric moisture was the subject of a communication by Prof. D. S. Martin to the New York Academy of Sciences. The phenomenon was observed in dark-blue flowers of this plant, and it was found to occur under two aspects, viz., 1. A reddening of the general blue color when the air is charged with moisture; and, 2. The production, by drops of rain, of sharply-defined spots upon the blue, which are at first red, and then bleach to white. Prof. Martin was led to examine the subject experimentally, by testing the flowers with acids and with ammonia-water. The result was such as was to have been expected with vegetable blues, that is to say, the ammonia had no effect, while the solutions of acids (oxalic, tartaric, and carbolic) produced the red coloring easily. Tried upon the pink variety of flowers, the acids had no effect, and the ammonia produced a strong blue color, which ere long passed away by evaporation. It is therefore evident, the author concludes, that this effect is due to some acid substance dissolved or absorbed by atmospheric water. If the latter is diffused through the air without precipitation, a general reddening of the blue corollas appears; if it falls upon them as rain and stands for a while, every drop produces a sharp spot that passes from red to white.

Barff's Method for preserving Iron.—A method proposed by Prof. Barff for preventing the corrosion of iron consists in producing upon the surfaces of the iron articles to be protected a coating of the black or magnetic oxide of iron. This he does by raising the iron articles, in a suitable chamber, to a temperature of from 500° to 1,200° Fahr., and then passing steam into this chamber, keeping the articles for five, six, or seven hours, as the case may be, at that temperature, in an atmosphere of superheated steam. At a temperature of 1,200° Fahr., and under an exposure to superheated steam for six or seven hours, the iron surface becomes so changed that it will stand the action of water, even though it be impregnated with the acid fumes of the laboratory. When the process is carried on at a lower heat, the iron articles will resist any amount of moisture with which they may come in contact in a house or building; but they will not permanently resist the action of the weather out-of-doors. The reason of this is, that only a thin film of the