OF NATURAL PHILOSOPHY. 57 



serious and distressing evil. In needle manu- 

 factories the workmen who point the needles are 

 constantly exposed to excessively minute particles 

 of steel which fly from the grindstones, and mix, 

 though imperceptible to the eye, as the finest dust 

 in the air, and are inhaled with their breath. The 

 effect, though imperceptible on a short exposure, 

 yet, being constantly repeated from day to day, 

 produces a constitutional irritation dependent on 

 the tonic properties of the steel, which is sure to 

 terminate in pulmonary consumption ; insomuch, 

 that persons employed in this kind of work used 

 scarcely ever to attain the age of forty years. * In 

 vain was it attempted to purify the air before its 

 entry into the lungs by gauzes or linen guards ; the 

 dust was too fine and penetrating to be obstructed 

 by such coarse expedients, till some ingenious per- 

 son bethought him of that wonderful power which 

 every child who searches for its mother's needle 

 with a magnet, or admires the motions and arrange- 

 ment of a few steel filings on a sheet of paper held 

 above it, sees in exercise. Masks of magnetized 

 steel wire are now constructed and adapted to the 

 faces of the workmen. By these the air is not 

 merely strained but searched in its passage through 

 them, and each obnoxious atom arrested and re- 

 moved. 



(47.) Perhaps there is no result which places in 

 a stronger light the advantages which are to be 

 derived from a mere knowledge of the usual order 

 of nature, without any attempt on our part to modify 

 it, and apart from all consideration of its causes, 



* Dr Johnson, Memoirs of the Medical Society, vol. v. 



