572 



NATURE 



[October 26, 191 1 



quid, and is subjected to the cooling effect of the expand- 

 11. and this constricts his cutaneous vessels and pre- 

 desaturation of the peripheral parts. The caisson 

 : ought to be decompressed in an air-lock which is 

 comfortably warmed, and he ought to exercise himself hard 

 in order to keep up the circulation and pulmonary ventila- 

 tion, and so hasten di saturation. 



Haldane thinks that the body of man is about half- 

 saturated in one hour, and about saturated in four hours. 

 Bornstein says six or seven hours are required for satura- 

 tion of the fat. Greenwood and I found that the urine 

 secreted by the kidney is about saturated after ten minutes' 

 expusure to four atmospheres. About twenty minutes were 

 occupied in reaching this pressure. On decompression of a 

 saturated animal the viscosity of the colloidal blood pre- 

 vents the formation of bubbles under a certain difference 

 of gas pressure. It is found by experience that it is safe 

 to decompress men in a minute or two from two atmo- 

 spheres to one. Since the volume of a gas is halved at 

 two atmospheres, made one-fourth at four atmospheres, 

 one-eighth at eight atmospheres, and the volume of a 

 bubble is doubled on lowering the pressure from eight to 

 four, six to three, four to two, or two to one, Haldane 

 concluded it was safe to come rapidly from four to two, 

 six to three, or eight to four atmospheres. The super- 

 saturated tissues then give nitrogen to the blood, and the 

 blood to the lungs, and the nitrogen escapes without 

 bubbling at the half-pressure stage, where a long pause is 

 given. Successive stages may be given when required to 

 secure the desaturation of the body, each stage by pro- 

 ducing a safe degree of supersaturation accelerating the 

 outgiving of the dissolved nitrogen. The stage method of 

 decompression initiated by Haldane, and adopted bv the 

 Admiralty, has an advantage over the uniform in that it 

 prevents the further, and perhaps dangerous, saturation of 

 the slow parts. Supposing a diver had been for half an 

 hour at six atmospheres pressure ; if he were decompressed 

 on the old plan, slowly and uniformly, his fat would be- 

 come further saturated up to five atmospheres while he 

 was being decompressed from six to five atmospheres. On 

 the other hand, if he is decompressed rapidly from six to 

 three, the further saturation of the fat at pressures above 

 three atmospheres is altogether prevented. The stage 

 method is of value to divers, who go down for short periods 

 and do not work very hard, as it prevents the saturation 

 of slow parts and hastens the period of decompression. 



Caisson workers who do four to eight hours' shift are 

 practically saturated ; but they, too, are best decompressed 

 by the stage method, because it accelerates the outgiving 

 of the nitrogen by producing a safe degree of super- 

 saturation of the blood. The safety is greatly enhanced if 

 hard muscular work is done during the pauses. This can 

 be effected by having a series of air-locks, and making the 

 men walk, or better, climb, between each. In the^East 

 River tunnels this method was tried with good results — 

 (1) +40 to +29 lb. in five minutes ; (2) ten minutes walk- 

 ing in +29 lb.; (3) +29 to +12J lb. in eight minutes; 

 (4) ten minutes walking in + 12J lb. ; (5) + 12J to +0 in 

 fifteen minutes. Lengths of tunnel were arranged between 

 locks for walking in. Total time, forty-eight minutes. 

 1 he Admiralty table enforces ninetv-seven minutes for this 

 pressure. 



As there were i-6o per cent, cases of " bends " and no 

 serious ones, the Admiralty time is demonstrated to be un- 

 necessarily long. This is particularly so if hard work is 

 done during decompression, for the same amount of 

 nitrogen would be expelled in about one-fifth of the time 

 as during rest. 



Greenwood and I have tested the stage method on pigs, 

 which are more like men in shape, diet, and habit than 

 goats— the animals used in the investigations condun. 

 the Admiralty Committee. It appears from our results 

 fairly safe to decompress even fat pigs from six atmo- 

 spheres to ?i atmospheres in about ten minutes, and then 

 after a pause of ij hours from 2J to one atmosphere in 

 twenty minutes. The pigs slept quietly in the warm 

 caisson and never moved, and, being fat, were very un- 

 favourable subjects. One death and no of ill- 



ness occurred among forty-seven pigs weighing 50 to 

 100 lb.; oni : three slight cases among nil 



ing 39 to 57 lb. A similai 

 2 191, VOL. 87 



fat pigs from seven atmospheres, allowing 105 to 120 

 minutes interval at 2^ atmospheres, gave unfavourable 

 results, seven deaths and one severe case — among twenty- 

 seven pigs weighing St to 115 lb. Only one pig out of all 

 showed any symptoms after reaching the stage at 23 atmo- 

 spheres. At these very high pressures there is great risk 

 unless time enough is given, and plenty of exercise taken 

 during the pause. 



For pressures up to four atmospheres the method 

 employed by Mr. Vapp at the East River Tunnel is 

 evidently a very good one. For pressure two to three 

 atmospheres it is an advantage to do work immediately 

 after decompression, supposing work cannot be provided 

 between two air-locks (Bornstein). At the Greenwich 

 Tunnel, now being built, the men climb the shaft, 60 feet 

 high, after decompression, and since I made the suggestion, 

 and the engineer, Mr. E. H. Tabor, carried it out, the 

 number of cases of '* bends " has dropped from 1 in 94 

 to 1 in 240 man-shifts. For higher pressures it would not 

 be safe to take exercise after ; it ought to be taken during 

 decompression and the pauses between the stage decom- 

 pressions. The importance of this cannot be insisted on 

 too much. Exercise during decompression is the simplest 

 means of rendering compressed-air work safe and of keep- 

 ing the period of decompression of a reasonable length. 



The question of the length of shift desirable has been 

 much discussed. Long shifts of eight hours are found to 

 give more illness than shifts of, say, one to two hours. 

 Every practical caisson engineer agrees to that. Divers are 

 decompressed in a few minutes from high pressures (five 

 to six atmospheres) with comparative immunity if they 

 have been down for only a few minutes. Cases of illness 

 occur when they exceed their stay, or after a succession 

 of dives, each of which helps to saturate slow parts and 

 increases the fatigue of the diver. The Admiralty table 

 fixes the period spent at the bottom so as to prevent satura- 

 tion of " slow " parts and shorten the period of decom- 

 pression. The descent is hastened for the same reason. 

 It is quite safe to descend to 200 feet in two minutes ; slow 

 descents only increase risk by increasing the saturation of 

 the body. In the matter of the caisson workers at the East 

 River Tunnel, two three-hour shifts per diem, with three 

 hours'' rest between, gave 1-07 per cent, cases, and one 

 eight-hour shift 062 per cent, cases. The men are so far 

 saturated in three hours of hard work that doubling the 

 decompressions is worse than extending the shift to eight 

 hours. As bubbles may persist for a long time in the 

 tissues, and may act as starting points for the formation 

 of other bubbles, it is wise to give long intervals of time 

 between shifts ; also in a short interval slow parts may not 

 become desaturated. Haldane has suggested the men 

 should return to a " purgatory " chamber, say at two 

 atmospheres, and eat their dinner and rest there in the 

 mid-period of an eight-hour shift, and again at the end 

 of the shift, when, while waiting for decompression to one 

 atmosphere, they could wash, change their clothes, and 

 have some hot coffee to stimulate the circulation. In any 

 large tunnel works such a chamber could be easily con- 

 structed out of a section of the tunnel. This would suffice 

 for stage decompression, and would give excellent results if 

 the men could be persuaded to take exercise in it, or be 

 given oxygen to breathe before decompression to one atmo- 

 sphere. 



The quickest method of desaturating the body is to 



" wash " the nitrogen out by breathing oxygen for a few 



minutes before and during decompression. The only ques- 



ihe safety of this proceeding, for high concentrations 



nison. 



Oxygen Poisoning. 



(1) I have found that all kinds of animals, worms, snails, 

 flies, spiders, frogs, &c, are instantly convulsed and killed 

 by exposure to fifty atmosphere: oxygen. (2) The frog's 

 heart beats, nerve conducts and muscle contracts for some 

 time in fifty atmospheres oxygen, but there is evidence of 

 progressive diminution in functional power; the muscles 

 behave like a fatigued muscle. (3) Mice exposed to ten 

 atmospheres oa rown into tetanic spasm, and on 



being decompressed continue to be convulsed by a touch. 

 Bubbles of oxygen are to be then found in the central 



