iads of floating particles can be seen glistening in the stream of light. Th

and num

es that people

ll never forget my rapt astonishment the day I first numbered the dust in the lectur

air is diluted with a fixed large quantity of dustless air (i.e. air that has been filtered through cotton-wool). The mixture is allowed to be saturated with water-vapour. Th

re 1 cubic inch. This observing table is divided into 100 equal squares, and is highly polished, with the burnishing all in one direction, so that during the observations it appears dark, when the fine mist-particles glisten opal-like with the reflected light in order that they may be more easily counted. The tube to which the silver table is attached is connected with two stop-cocks, one of which can admit a small measured portion

ater-vapour takes place on the dust-particles. The observer, looking through the magnifying-glass upon the silver table, sees the mist-particles fall like an opal shower on the table. He counts the number on a single square in two or three places, striking an average in his mind. Suppose the average number upon a single square were five, then on the whole table there would be 500; and these 500 particles of dust are those which float

de of the room, due to the quantity of smoke from the passing trains. Dr. Aitken has counted in 1 cub

e for ordinary purposes. It is so constructed that, when the different parts are unscrewed

inches of air pass in and out at every breath, and adults breathe about fifteen times every minute. But the warm lung-surface repels the colder dust-pa