THE AIR WE BREATHE. 21 



it would be injurious, by the innumerable fine hairs or 

 cilia which cover the air-passages, and which, as they con- 

 stantly wave upwards, filter the air very effectually ; but 

 when the strain is too great and prolonged, the hairs cease 

 to act, the membrane of the air-passages becomes inflamed, 

 and bronchitis or asthma follows. 



The dust of coal-mines and that caused by grinding, 

 especially steel-grinding, and the polishing of pearl-buttons, 

 marble, &c., particularly where emery is used ; also the dust 

 in potteries and china-works ; the organic dust and fluff of 

 shoddy- and flax-mills ; as well as that arising from the sort- 

 ing of type, are all injurious and some of them fatal in their 

 effects upon the air-passages and lungs, which the hairs 

 are quite unable to protect. A seedsman once complained to 

 Professor Tyndall that his men were made quite ill during 

 the busy season by the irritation produced by the dust from 

 the seeds, and gladly accepted his suggestion that they 

 should be provided with respirators made of cotton-wool 

 tied up in muslin, which filtered the air so perfectly that no 

 further complaints were heard. 



The "black lungs" of colliers are well known, and 

 stony dust is found deposited in the lungs of stone-masons, 

 but, under ordinary circumstances, the natural filtering 

 apparatus is quite effectual, the particles being arrested by 

 the hairs above mentioned and then sent back into the air 

 by the expired breath. The air which we breathe out at the 

 end of an expiration is so absolutely free from dust, that, if 

 breathed across the track of an electric beam, the latter will 

 be pierced by an intensely black hole, for the reasons 

 already given. 



