370 Mr. Leonard Hill [May 25, 



run together to form visible bubbles. We find colloidal solutions 

 form super-saturated solutions after exposure to high pressures. 

 After decompression the gas does not come off until the solutions are 

 shaken. I surmise that the gas molecules are tacked on to the 

 colloidal particles, and that skins of proteid antagonise the union of 

 the gas molecules with each other. 



The gas bubbles tend to collect in the veins, as the blood travels 

 quickly through the arteries and slowly in the veins. The higher 

 blood pressure in the arteries, I think, cannot appreciably influence 

 this. It is only when the gas in the veins becomes sufficient in 

 amount to produce foam in the heart, or when gas bubbles block 

 up arteries of vital import, that grave symptoms arise. The place 

 where bubbles in the arteries must always produce serious results 

 is the central nervous system. In the liver, kidneys, muscles, 

 fat, etc., bubbles may embolise small arteries and produce no grave 

 effect, but in the spinal cord the interruption of the blood supply to 

 any group of cells or tract of fibres, is evidenced at once by pain and 

 auEesthesia, spasm and paralysis. In the medulla oblongata arrest of 

 the circulation will stop respiration, and bubbles lodging there may 

 produce immediate death. Lodging in the arteries of the great 

 brain, bubbles may produce hemiplegia, aphasia, blindness, or mental 

 disturbance. 



The explanation of the idiosyncrasy of different animals in this 

 respect is not altogether clear. Small animals, such as rats and mice, 

 and young animals generally, escape owing to the rapidity of respira- 

 tion and circulation, which frees their bodies from nitrogen. But 

 among larger animals and among men some are affected and otliers not. 

 We can look for an explanation in the varying state of that colloidal 

 solution, the blood, in the varying vigour of the circulation and 

 respiration and the effect of fatigue, in vaso-motor changes which 

 alter tlie relative volume of circulating blood in viscera and muscles, 

 and possibly to a minor degree in the fermentative processes going on 

 in the alimentary tract. The young man who is in perfect health, 

 with powerful heart and deep respiration, can expel the dissolved 

 nitrogen from his lungs far more rapidly than the old, the intemperate, 

 or one who is over-fatigued by excessive labour. The records of 

 caisson works seem to show that men under 20-25 years escape ; that 

 long shifts increase the number of cases ; that men who work the air- 

 locks, passing material through, and undergoing frequent and short 

 lasting compression and decompression, are not affected. The longer 

 the shift the more complete the saturation of the body ; the higher 

 the pressure the greater tlie risks and the graver the symptoms. The 

 records show that ])ractically no cases occur with a pressure l)elow 

 \\ atm., even though the decompression period be made only a 

 minute or two. 



At the Rotherhithe tunnel, now building, the decompression 

 period is :■> minutes, and the pressure + 22 lb. No cases of any 



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