EFFECTS OF CHANGES IN AIR BREATHED 1231 



from the high pressure to the ordinary atmospheric pressure should 

 be carried out gradually, so as to give the blood-plasma, supersaturated 

 with nitrogen, time to get rid of its excess of nitrogen without the forma- 

 tion of bubbles. 



OTHER GASES. Hydrogen and methane are, like nitrogen, in- 

 different gases. They may be respired if mixed with 20 per cent, of 

 oxygen, and either of the gases may be used instead of nitrogen to 

 dilute the oxygen that we breathe, without harm or inconvenience. 



Carbon monoxide is rapidly poisonous by its action on the red 

 corpuscles. It combines with haemoglobin, forming CO- haemoglobin, 

 a compound which is much more stable than oxyhsembglobin. The 

 blood is therefore deprived of its oxygen carrier, and the animal dies of 

 asphyxia. We have seen, however, that the displacement of oxygen 

 by CO is not absolute, but only relative. Hence, although the avidity 

 of CO for haemoglobin is 140 times that of oxygen, we can convert the 

 CO back into oxyhaemoglobin by increasing the mass influence of the 

 oxygen. This may be done by giving the poisoned animal pure 

 oxygen to breathe, or even oxygen under pressure. In pure oxygen 

 at a pressure of two atmospheres an animal can breathe and live, even 

 though the whole of its haemoglobin is converted into CO-hsemoglobin, 

 the amount of oxygen which is simply dissolved by the blood-plasma 

 being sufficient at this pressure for the respiratory needs of the animal 

 (Haldane). 



Other gases which have special poisonous properties are hydro- 

 cyanic acid, sulphuretted hydrogen, phosphuretted hydrogen (PH 3 ), 

 arseniuretted hydrogen, &c. 



IRRESPIRABLE GASES are those which are so irritating that 

 they produce spasm of the glottis. Such are ammonia, chlorine, 

 sulphur dioxide, nitric oxide, and many others. 



VENTILATION 



A point of practical importance is the securing to each individual 

 of sufficient fresh air, so that he may always have a plentiful supply 

 of oxygen, and may be relieved of his waste products. It is found that 

 a dwelling-room becomes unpleasant and stuffy when the percentage 

 amount of C0 2 has reached 0-1 per cent. This stuffiness is supposed 

 to be due to organic exhalations from the skin, lungs, and alimentary 

 canal, some of which have a poisonous effect, giving rise to headache 

 and sleepiness. Since these cannot be measured, it is taken as a 

 cardinal rule in ventilation that the amount of C0 2 should never rise 

 above 0-1 per cent. 



Since in questions of ventilation we have generally to deal with 

 trades in which the metric measure is not used, it may be convenient 



