g 



THE WORK OF DUST. 651 



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 con- 

 tents 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 bot- 

 tom. 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 ex- 

 ample, 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 in- 

 cessantly 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 compo- 

 sition 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 



