642 EVENING DISCOURSES. 



Influence of Fatness. 



As the fat holds five or six times as much nitrogen in solution as the blood, 

 it saturates and desaturates slowly. 



J. F. Twort and I have found 35'55 per cent, of nitrogen dissolved in olive oil 

 which had been exposed to 7£ atmospheres. The risk of exposure to compressed 

 air varies with the fatness of the animal (Boycott and Damant). Greenwood 

 and I have found fat pigs weighing 100 to 120 lb. are more susceptible than 

 smaller pigs 50 to 60 lb. The bubbles once set free in the subcutaneous fat of 

 pigs may stay there for days after decompression, as we have found to our cost, 

 for it has seriously damaged the sale of the animals to the butcher, since the fat 

 does not bleed wliite, but remains pink and mottled. All the results prove that 

 fat men should be excluded from compressed air work at pressures over 2 atmo- 

 spheres absolute. 



The varying percentage of fat in the blood, chyle, and liver must be an im- 

 portant factor in the evolution of bubbles in the blood. The less fat in the 

 food eaten by caisson workers the better. 



Ventilation and Illness. 



Much has been made of the impurity of the air as a contributory cause of 

 caisson sickness, in particular, of the percentage of C0 2 . The ventilation of the 

 tunnels built by the London County Council under the Thames have been car- 

 ried out at enormous and needless expense, in order to keep the CO;, percentage 

 down to a very low level. The work of the English physiologists is against this 

 view. Divers generally work with 1, 2, or even 3 per cent, of an atmosphere 

 CO., in their helmets. We have exposed ourselves to 3 to 4 per cent, of an 

 atmosphere CO., without untoward results, beyond increased frequency of respira- 

 tion — which prevents any increased concentration of CO., in the body. 



Recently I have carried out many experiments on students sealed up in a small 

 air-tight chamber, and found, as Haldane has, that it is the heat, moisture, and 

 stillness of the air which cause discomfort and fatigue, and not the excess of 

 CO,, or deficiency of oxygen in the air breathed. The putting on of powerful 

 electric fans by whirling the air and cooling the body gives very great relief, even 

 when there is 4 to 5 per cent, of CO, in the chamber. 



In open-air treatment the coolness and movoment of the air are the essential 

 qualities which promote health by stimulating the activity, the metabolism, and 

 nervous well-being of the body. 



Hot, moist, still air causes fatigue by taxing the cooling mechanism of the 

 body; blood is sent to the skin to be cooled which ought to be going to muscle 

 and brain. Fatigue increases the danger of decompression by making the circu- 

 lation and respiration less efficient. The heat causes more blood to come to the 

 skin and a more complete saturation with nitrogen there. The cold in the decom- 

 pression chamber — due to expansion of the air — -causes vaso-constriction and re- 

 pels the blood from the skin and so stops its desaturation. We have lost pigs by 

 taking them from the warm caisson into the cold air. 



Over-hot and moist — that is, under-ventilated — caissons have, therefore, a 

 higher morbidity. To secure efficient work the wet-bulb temperature must be 

 kept below 75° F. (Haldane). The men should not pass from a warm caisson to 

 a cold air-lock and a cold outside world. They should go through a warm lock 

 to a warm room. 



Hot moist atmospheres are very disadvantageous to health and work. If the 

 wet-bulb temperature is high in the caisson, the current of air should be in- 

 creased or electric fans used to cool the workers. Electric fans have enormously 

 increased the efficiency and health of Europeans in the tropics. An excess of 

 CO., in the air-lock, or diver's helmet, during decompression is favourable, as it 

 increases the pulmonary ventilation and the outbreaking of nitrogen. Haldane 

 advises the air-pump to be slackened purposely. There is no harm in breathing 

 1 or even 2 per cent, of CO,. 



Methods of Decompression. 



The safety of compressed-air workers depends on the relation of the period 

 of decompression to that of compression. 



The period of the saturation or desaturation of the body with nitrogen de- 

 pends on the relation between the circulating volume of the blood and the 



