Page:Popular Science Monthly Volume 47.djvu/439

Rh of American corundum was six hundred and forty-five tons. Unlike corundum, emery consolidates in large masses. It does not, indeed, form continuous beds of great extent, but its discontinuous masses and.veins sometimes contain hundreds of tons. The emery-bearing locality in Westchester County, New York, is a strip from one half to three fourths of a mile in width and from five to six miles in length. The place in which the largest openings have been made, and which has excited the most interest, is on a part of a summit about three miles from Peekskill and seven hundred or eight hundred feet above tide level. It overlooks, on the one side, the valley of the Croton, whose stream is invisible, and, on the other side, the Hudson. On the north and northeast of the emery belt are outcrops of granite. South of it lies the common marble of Sing Sing; still farther south, at Spuyten Duyvil, occur the oldest of the Laurentian gneisses; and still farther south, on Manhattan Island, the mica schist. The emery is, however, all immediately associated with a hornblende rock. Large masses of emery are seen projecting above the surface. These are delusive, and those which hold out a large promise are sometimes found to extend only one or two feet underground and to yield only from five to twenty tons. Such masses are usually surrounded by soft, reddish earth.

Substitutes for White Lead.—Only two substances at present manufactured are regarded by Mr. A. P. Laurie as satisfactory substitutes for white lead in painting—sulphate of lead and oxide of zinc. Sulphate of barium has hardly any covering power, and sulphide of zinc, though remarkable for covering power, has not proved, as at present manufactured, a durable pigment. Oxide of zinc, though deficient in covering power, is remarkably white, and preserves its color in impure air. Sulphate of lead is in the market in two forms—sublimed sulphate, which is prepared directly from galena; and precipitated sulphate, ground by Freeman's patent with oxide of zinc, and sold as Freeman's white. Sulphate of lead prepared by sublimation has much more covering power and is much denser than precipitated sulphate. Another pigment sold as a harmless white lead is prepared in a similar way by grinding together oxide of zinc and sulphate of barium. In quantity of oil required the substitutes named compare well with white lead, some taking a little more and some a little less, except oxide of zinc, which takes a very large quantity. In the matter of susceptibility to impure air, they all have a distinct advantage over white lead. Zinc oxide is not at all affected, and the sulphate is very slightly affected unless the gas is in very large quantities and the paint is wet. In durability under outdoor exposure they are not better than white lead, except that oxide of zinc remains white. In their appearance in oil they differ considerably from white lead, being thin and stringy instead of stiff and firm, and this is against them. But Mr. Laurie does not find that when thinned down they seem to differ appreciably from lead carbonate in ease of working. In their effects on health, oxide of zinc is harmless. Sulphate of lead is not absolutely insoluble in very weak hydrochloric acid, and may therefore be slightly soluble in the stomach and to some extent poisonous; but the author does not believe that under ordinary conditions of manufacture or use it would produce lead poisoning.

The Dangerous Proportion of Carbonic Acid.—Of the power of carbonic acid to smother, Prof. F. Clowes, of Nottingham, England, ascertained that the flames of candles, oil, paraffin, and alcohol are extinguished by air containing from thirteen to sixteen per cent of carbonic acid. The flame of coal gas requires the presence of at least thirty-three per cent of the extinguishing gas, while the flame of hydrogen requires fifty-eight per cent. Concerning the proportion of carbonic acid mixed with water that can be breathed with impunity, the statements of different observers are conflicting. Prof. Clowes finds ten per cent more than is required to extinguish a candle flame respirable, while Dr. Haldane, of Oxford, estimates that air containing twenty per cent of carbonic acid can not be breathed, even for a minute, without serious consequences; even five per cent, he claims, caused serious distress of body and mind, while any proportion higher than ten per cent produced distinct poisonous effects.