POPULAR MISCELLANY. 



633 



their limit of elasticity, whereby the mole- 

 cules, brought to a state of flow, glide over 

 each other, and assume a wholly new po- 

 sition 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 elas- 

 ticity. 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 con- 

 siderable time. It is also well to apply a 

 gradually-increasing weight. 



Properties of White Paint. As the re- 

 sult of hundreds of experiments in carbon- 

 ate of lead and hydrate of lead, both sepa- 

 rately and mixed, Messrs. Wigner and Har- 

 land, of the British Society of Public Ana- 

 lysts, 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 be- 

 neath, must consist of an admixture of hy- 

 drate 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 hy- 

 drate. 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 pos- 

 itive chemical compound has been formed, 

 and the seventy-five per cent, or there- 

 abouts, of carbonate of lead present has 

 been dissolved in the chemical compound, 

 and so a paint has been formed which pos- 

 sesses 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 pos- 

 sessing characters somewhat different from 

 a mere emulsion, it is vain to expect that 

 they can equal good white-lead. 



Cliange of Tint in Flowers. The change 

 of tint in flowers of Ipomoa purpurea (morn- 

 ing-glory) under the influence of atmos- 

 pheric moisture was the subject of a com- 

 munication by Prof. D. S. Martin to the 

 New York Academy of Sciences. The phe- 



nomenon 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 produc- 

 tion, 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 experimen- 

 tally, by testing the flowers with acids and 

 with ammonia-water. The result was such 

 as was to have been expected with vege- 

 table blues, that is to say, the ammonia had 

 no effect, while the solutions of acids (ox- 

 alic, tartaric, and carbolic) produced the red 

 coloring easily. Tried upon the pink varie- 

 ty 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 sub- 

 stance dissolved or absorbed by atmospher- 

 ic water. If the latter is diffused through 

 the air without precipitation, a general red- 

 dening 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. 



Barflfs Method for preserving Iron. A 



method proposed by Prof. Barff for pre- 

 venting 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 suit- 

 able chamber, to a temperature of from 

 500 to 1,200 Fahr., and then passing 

 steam into this chamber, keeping the arti- 

 cles for five, six, or seven hours, as the case 

 may be, at that temperature, in an atmos- 

 phere of superheated steam. At a tempera- 

 ture 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 ac- 

 tion of the weather out-of-doors. The rea- 

 son of this is, that only a thin film of the 



