The solid matter floating in our atmosphere is every day becoming of greater and greater interest as we are gradually realising the important part it plays in the economy of nature, whether viewed as to its physical, physiological, or meteorological aspects. One fundamental point on which we have at present very little information of anything like a definite character, is as to the number of solid particles present in our atmosphere. We know that they are very numerous, and it seems probable that the number varies under different conditions of weather; but what number of particles are really present under any conditions, and how the number varies, we have at present very little idea. In this field of research the physiologists are far in advance of the physicists, as they have devised means of counting the number of live germs floating in our atmosphere, and already we have a good deal of information as to how the number varies under different conditions.