636 EVENING DISCOURSES. 



I have noted that the same false views are even now put forward in the 

 daily press to explain the symptoms, due to the rarefaction of the air, endured 

 by aeroplanists. The sickness of high altitudes suffered by mountain climbers, 

 balloonists, and aeroplanists has nothing to do with the mere mechanical effect 

 of the lowering of barometric pressure. In an atmosphere enriched with oxygen 

 U. Mosse has endured a lowering of barometric pressure until he could span 

 the height of the column of mercury in the barometer with his hand. Oxygen 

 want, due to the rarefaction of the air, is the prime cause of altitude sickness. 

 At an altitude of 18,000 feet, where the barometric pressure is halved, a man, 

 filling his lungs with air, takes in only half the weight of oxygen which he 

 takes in at sea-level. His respiratory and circulatory organs can scarcely work 

 hard enough for the body to get enough oxygen. 



That mere mechanical pressure, uniformly applied, is of no importance to 

 living matter is shown by the existence of life in the greatest depths yet 

 sounded, where the superincumbent pressure may equal two, three, and even 

 five miles of water. By means of a small chamber and hydraulic pump and 

 lantern I project the shadow of the frog's heart beating in a suit- 

 able salt solution at a pressure of 2,000 lb. (133 atmospheres), equivalent to 

 a depth of nearly a mile of water. Regnard has compressed living aquatic 

 animals, frogs' muscles, &c, to 500 and even 1,000 atmospheres, and has found 

 at the highest pressures the tissues become stiff and take up water, and life is 

 destroyed. His experimental results and those of the deep-sea soundings 

 (Challenger Reports) are in contradiction. Regnard's experiments require repeti- 

 tion, with careful attention to the chemical composition of the water in which 

 the living matter is compressed. 



I refute the mechanical theories of compressed-air illness by this experiment : 

 A frog's web is stretched over the glass window of the small pressure chamber, 

 and is illuminated by the arc light, so that the circulation of the blood is pro- 

 jected on the screen. The circulation remains unchanged when the pressure is 

 rapidly raised to 20 or even 50 atmospheres. 



Manometric records of blood pressure taken from mammals enclosed in a 

 pressure chamber, or from man, show no noteworthy change when the pressure 

 is raised to 3 atmospheres. Similarly I can show that a frog's heart or muscle 

 contracts normally when suddenly submitted to a pressure of air equal to 

 50 atmospheres. After a time the contraction languishes, but that is not due 

 to the pressure per se, but to poisoning by the high pressure (concentration) 

 of oxygen. The pressure uniformly applied has no mechanical effect on the living 

 protoplasm. 



The Evolution of Diving Apparatus. 



The use of compressed air for submarine work was a matter of slow develop- 

 ment, owing, not to lack of invention, but to want of efficient air-pumps and 

 flexible tubes. The naked divers had a barrel, or bell-shaped vessel, standing 

 on a tripod, lowered down to them full of air, to which they could return and 

 breathe the air within every minute or two. They also chewed pieces of sponge 

 dipped in oil, probably because swallowing inhibits the respiratory centre and 

 checks the desire to breathe. One of the oldest inventions is that of a pipe 

 conveying air from the surface to the mouth of the diver. Such a device cannot 

 be used at any depth, because the body is pressed upon by the water plus the 

 atmospheric pressure, while the lungs are exposed to the atmospheric pressure 

 alone. This makes breathing difficult and dangerously congests the lungs with 

 blood, as I can demonstrate by this model. The cupping glass also demonstrates 

 the congestive effect produced by lessening the atmospheric pressure at one part 

 of the body only. Bernouilli (seventeenth century) formulated the correct theory 

 that the diver must be supplied with air at the pressure of the water surrounding 

 him. In the older inventions the air escaped from under the helmet and only 

 the head was dry. The air pressure in the modern diving-dress (invented by 

 Siebe), regulated by a valve in the helmet, keeps the water from entering at 

 the wrist cuff, and the whole body is kept dry and warm and equally com- 

 pressed. I demonstrate the modern diving dress which Messrs. Siebe, 

 Gorman & Co. have lent me for this lecture. The pressure produced by the 

 pump must keep up to that of the water as the diver descends, so long as he 

 does not fall down. He can descend rapidly, e.g., 100 feet in two minutes, but 



