an organic nature. It is seen to consist of countless motes, rising, falling, or gyrating, although it is impossible to follow any of them with the eye for longer than a fraction of a second. We conclude that their weight exceeds but very slightly that of the air; and, moreover, that the atmosphere is the seat of multitudes of minute currents, assuming all kinds of directions. One day last June, from the top of Eiffel's Tower in Paris, I amused myself by throwing an unfolded newspaper over the railing round the summit of the tower. At first it fell slowly, carried away by a light breeze; but presently it rose, and, describing a curve, began again to fall. As it was vanishing from sight, the paper seemed to me as if arrested now and then in its descent, perhaps undergoing again a slight upheaval. Here was, indeed, a gigantic mote floating in the atmosphere, and subject to the same physical laws, though on a larger scale, as those delicate filaments of dust we see dancing merrily in a sunbeam.
It is difficult to say how much of the dust present in the air may become a source of disease, and how much is innocuous. Many of the motes belong to the class of micro-organisms; and experiments show how easily these micro-organisms or sources of infectious diseases can reach the lungs, and do mischief if they should find a condition of the body on which they are able to thrive and be reproduced. Atmospheric motes, although it has been shown that they are really deposited in the respiratory organs, do not accumulate in the lungs and air-passages, but undergo decomposition and disappear in the circulation. Smoke, which is finely divided coal-dust, is clearly subjected to such a destructive process; otherwise the smoky atmosphere of many of our towns would soon prove fatal, and tobacco-smoke would leave a deposit interfering seriously after a very short time with the process of respiration. Dust, however in its physical aspect is very far from being always innocuous, and many trades are liable to suffer from it. The cutting of chaff, for horses' food, is one of the most pernicious occupations, as it generates clouds of dust of an essentially penetrating character. Persons engaged in needle manufacturing and steel-grinders suffer much from the dust of metallic particles. Stone-cutters, and workmen in plaster of Paris, coal-heavers, men engaged in the manufacture of cigars and rope, those employed in flour-mills and hat and carpet making, are liable to suffer from dust. A number of methods have been adopted, more or less successfully, to rid these trades of the danger due to this source. I observed many years ago that charcoal has the power of retaining dust in a remarkable degree, and having had respirators made of it, found them very effective in preventing dust reaching the lungs.
Micro-organisms—dust-like particles capable of cultivation or
