384 SCIENCE PROGRESS 



dust from coal, or soft stone, and many other materials, has 

 no such effect, [at least in moderate quantities. An average 

 English coal-miner inhales very large amounts of coal-dust ; 

 but coal-mining is one of the healthiest of all occupations, in 

 spite of absence of light, exposure to extremes of temperature, 

 and the inhalation of air which is, chemically speaking, far 

 from pure. It just happens that the ordinary impurities — 

 moderate excess of C0 2 , CH 4 , dust from coal and shale, and 

 deficiency of oxygen — matter comparatively little from the 

 health point of view, whereas dust from hard stone matters a 

 great deal. 



I must now return to deep diving. The most formidable 

 danger presented by this occupation occurs only on the ascent 

 of a diver to surface. It has been known for long that divers 

 and other workers in compressed air are subject on their return 

 to surface to a variety of serious symptoms — in mild cases only 

 pains in the limbs or elsewhere (usually known among English 

 workers in compressed air as " bends ") ; in more serious cases 

 paralysis, either temporary or permanent, particularly of the 

 legs and bladder ; or, finally, symptoms of asphyxia, which may 

 prove rapidly fatal. The classical experiments of Paul Bert ! 

 proved, thirty years ago, that all these symptoms are due 

 to liberation of gas-bubbles in the blood or elsewhere within 

 the body ; and considerable further light has been thrown 

 on this subject by more recent work, including particularly 

 that of Catsaras, von Schrotter, 2 and Hill. It was on the 

 prevention, rather than on the pathology, of these symptoms, 

 known collectively as caisson disease, that the Admiralty 

 Committee concentrated its attention. The blood and body 

 tissues become saturated with gaseous nitrogen in accordance 

 with Dalton's Law during exposure to compressed air. On 

 rapid decompression this gas liberates itself in the blood as 

 bubbles, which cause local or general blocking of the circulation, 

 with corresponding symptoms. Paul Bert showed on animals 

 that in order to avoid this condition decompression must be 

 slow ; but the question, How slow ? had not been satisfactorily 

 answered in the case, at least, of man. The nearest attempt at 



1 La Pression Baromc'h'iqtce, 1878. 



2 Heller, Mager, and von Schrotter, Die Luftdruckerkrankungen, Vienna, 1900 ; 

 von Schrotter, Der Saurerstoff in der Prophylaxie und Therapie der Luftdrucker- 

 krankungen, 2nd ed., Berlin, 1906. 



