particles in the air. Pasteur had already begun an investigation in that direction. He filtered a measured quantity of air through gun cotton, which retained all the particles of dust. This was then dissolved in a mixture of ether and alcohol, and the solution was dried to a sheet of clear and transparent collodion, in which the particles could be observed under the microscope and counted. The chief purpose of this experiment was to secure the yeast germs in the air. A better process for counting dust is based on our experiment with the dustless flask, and, like that, was devised by Mr. John Aitken, in Edinburgh. A measured quantity of the air to be tested—say, about a hundredth part of the contents of the flask—is let into it. The counting is facilitated by this dilution. The air in the flask has been already saturated with moisture, while it has been compressed by forcing in some dustless air. A faucet is suddenly opened, when the air expands, is cooled by the expansion, and the vapor settles on all the dust particles, weights them, and causes them soon to sink to the bottom. The bottom of the flask is made of a bright silver or a glass plate, on which a network of square millimetres is scratched. On this network as many drops of water fall as there were dust particles, and they can be counted with a lens. The number of dust particles in a cubic centimetre of air is—in London, for example, even at the borders of the city, and when the wind is blowing toward it from without—nearly a quarter of a million. About the same number are found in the air of Paris, and half as many at the top of the Eiffel Tower. The air of the Alps is very much purer. On the top of the Rhigi there were about two hundred particles to a cubic centimetre, and a few less after a fall of rain. In the relatively pure air of mountain tops the breath is not condensed into a visible cloud, even in cold weather. As we descend and approach villages whose chimney tops are smoking, the accustomed breath clouds appear again. But a steam jet is visible everywhere, for perfectly dustless air is not found anywhere.
Dust is usually spoken of as something peculiar to the earth. It is, however, present in space. Our solar system has its dust atmosphere, although it is extremely thinly sown. Besides the large blocks of matter, the meteoric stones, meteoric dust is incessantly falling from space upon the earth. Attention was first directed to this fact in 1869, when a meteorite fell at Upsala, and a shower of black dust at the same time. The dust was collected, and exhibited the same composition as the meteorite—carbon and iron. Since then several falls of cosmic dust of identical composition have been observed where no meteorites were seen. The recent advance of celestial photography has furnished images of externally faint clouds floating in space. These clouds do not
