RESPIRATION 309 



was 2.7 per cent at the breathing level. With larger escapes of 

 gas this percentage could hardly be increased, as most of the gas 

 remained at the roof. The air at all parts of the rooms tested was 

 examined with a miner's safety lamp to see if the air ever became 

 explosive; but with such escapes as could be produced when 

 burners were not taken off, I never succeeded in obtaining an 

 explosive atmosphere even at the roof. It requires about 8 per 

 cent of lighting gas to render air explosive. 



These experiments had a very definite practical significance in 

 connection with the composition of lighting gas used for domestic 

 purposes : for it is evident that whether or not a dangerous result 

 will ensue from an escape of gas in a room will depend on how 

 poisonous the gas is, and not simply on the time during which the 

 escape continues. The poisonous action of lighting gas largely 

 diluted with air depends exclusively on the CO contained in it. 

 In every case of persons found dead in air containing lighting gas 

 the post mortem appearances are those of CO poisoning, and the 

 percentage saturation of blood as determined by the method de- 

 scribed in the appendix has turned out to be round 80, just as in 

 the case, referred to below, of miners poisoned by CO. Thus, 

 broadly speaking, the danger of poisoning from escape of lighting 

 gas depends on whether the air will be poisonous from CO when 

 less than 2 or 2.5 per cent of gas is present. 



Lighting gas as originally introduced is made by the distillation 

 of bituminous coal, and usually contains about 7 or 8 per cent of 

 CO. With 2 per cent of this lighting gas in the air there would 

 only be about 0.14 per cent of CO ; and this, though a formidable 

 percentage, would not, so far as known, produce fatal effects in a 

 healthy person, as the haemoglobin would, in all probability, not 

 become much more than about half -saturated. To judge from all 

 our present knowledge, and from the results of experiments on 

 animals, about 0.3 per cent would usually be needed to produce 

 death within a few hours. 



Excellent lighting gas can also be made by blowing steam 

 through incandescent coke or coal. The product is what is called 

 "blue" water gas consisting roughly of equal parts of hydrogen 

 and CO. This gives a very hot, though small, flame, and although 

 the flame by itself is "blue" and practically nonluminous, an ex- 

 cellent light is given when a properly adjusted mantle is used. On 

 the other hand the calorific value of a given volume of this gas is 

 very low as compared with ordinary coal gas ; and as the value of 

 gas depends mainly on the heating power of a given volume of it, 



