ATMOSPHERIC DUST. 179 



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 my- 

 self 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 under- 

 going 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 un- 

 dergo 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 mak- 

 ing, 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 char- 

 coal 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 



