from an eminent teacher. This method, were it applied to the whole working literature of education, would place the judgments of the best teachers at the service of all the people. Of the catalogue of this library the Bureau of Education is printing twenty thousand copies.
Thickness of Oil Films.—From experiments made in the Baltic Sea off Greifswald, Prof. Oberbeck, of the University of Greifswald, has found that the surface of water calmed by one litre of rape-seed oil or machine oil oscillates around nineteen thousand square metres, indicating that the thickness of the film is about one twenty-thousandth of a millimetre. The oil doubtless extends also in an imperceptible film outside of the circle of calm, whence the average thickness of this inner layer is probably even less. The author has made skillfully devised series of laboratory experiments to determine still more precisely the minimum thickness of a perceptible film, and found it to be two millionths of a millimetre. This is the same thickness as that which Lord Rayleigh found adequate to arrest the movements of camphor. Mr. Rontgen has also found that the vapor of ether striking upon oil scatters it till it is reduced to the same thinness. According to Herr Oberbeck, a film six times thinner is still coherent. If the quantity of oil is gradually increased the pellicle becomes more and more resistant, and of uniform thickness. When it reaches eighteen millionths of a millimetre, the oil collects in droplets which rise above the rest of the surface; and the film does not become uniform till enough oil has been poured on to equal the entire thickness of the droplets.
Advances in the Dairy Industry.—At the Dairy Building at the World's Fair there were daily demonstrations of the best modern practice in butter and cheese making. Prof. S. M. Babcock, of the University of Wisconsin, the chemist in charge, as part of his apparatus, employed the milk tester invented by him in 1890. This tester is used by adding to milk an equal quantity of sulphuric acid of 1·82 or 1·83 specific gravity. The mixture is poured into a series of glass bottles, each drawn out at the neck as a narrow and calibrated tube; the bottles, laid in an inclined position on a frame, are rotated 700 to 1,200 times per minute; the sulphuric acid separates the fat, and this fat, by centrifugal motion, is sent up into the calibrated tubes, where it is easily read off. This test places the dairy industry upon a business footing, and not only enables the proprietor of a butter or cheese factory justly to appraise the milk he buys, but also decides for the dairyman which of his cows is most profitable and which should be sent to the butcher. The importance of this simple and ready test is evident when we learn that in Wisconsin alone there are 1,700 butter and cheese factories. The Babcock tester is manufactured by some twenty firms in the United States, and by a firm in England and a firm in Germany. Due as it was to the experiments of a servant of a State, the device has not been patented. To this fact is in part due the wide sale of the tester; it is so simply manufactured that no costly patterns and plant are needed for its production; at retail the price is but eight to twelve dollars, according to size. To his forerunners in the task of fat testing Prof. Babcock declares his indebtedness. Mr. Short, of the University of Wisconsin, had invented an apparatus in which milk fat was saponified and driven forth by centrifugal motion; Prof. Patrick, of Iowa, employed, in a tester of his design, an acid instead of an alkaline combination. Uniting an idea from each of these devices, Prof. Babcock hit upon success.
Vegetarian Pedestrians.—The result of a pedestrian contest recently completed between Berlin and Vienna was a triumph for two vegetarian walkers, who came out a long way ahead of their carnivorous competitors. The fact corresponds with other evidence of the enduring power of non-meat-eaters. If there is one thing certain, says an English journal, remarking on the achievement, about the races that eat no meat, it is that they can march. "Thousands, probably scores of thousands of Sikhs and Hindostanees would have performed the German feat, and not have thought at the end of it that they had done anything wonderful; and they not only eat no meat, but they are the descendants of men who have eaten no meat for perhaps two thousand years. They have eaten wheat
