886 OF RESPIRATION. 



this proportion may rise much higher ; particularly when the respira- 

 tory movements are slowly and laboriously performed. Now in order 

 that the blood should be properly aerated, it is requisite that the air 

 should contain no previous impregnation of carbonic acid ; since the 

 diffusion of even a moderate percentage of that gas through the 

 inspired air, seriously impedes the exhalation of more. Thus it was 

 found by Messrs. Allen and Pepys, that, when 300 cubic inches of air 

 were respired for three minutes, only 28 J inches of carbonic acid (or 

 somewhat more than 9 per cent.) were present in it ; though the rate 

 of its production in a parallel experiment, in which fresh air was taken 

 in at each inspiration, was 82 cubic inches per minute, or 96 cubic 

 inches in three minutes. It appears from the experiments of Dr. Snow, 

 that the presence of carbonic acid in the atmosphere acts more delete- 

 riously on the system, in proportion as the normal quantity of oxygen 

 has been reduced ; and hence, that the substitution of carbonic acid for 

 oxygen by the respiratory process, vitiates the air far more effectually 

 than the introduction of a surplus of carbonic acid, the normal quantity 

 of oxygen being still present. He concludes from his experiments 

 upon the lower animals, that 5 or 6 per cent, of carbonic acid cannot 

 exist in an atmosphere respired by Man without danger to life ; and 

 that less than half this amount would soon be fatal, when it is formed 

 at the expense of the oxygen of the air. A still smaller proportion is 

 capable of producing very injurious results. Thus the discomforts 

 occasioned by the presence of a crowded audience in a church, lecture- 

 room, or theatre, which is not provided with sufficient ventilation, are 

 due in great part to the continued respiration of air, which becomes 

 loaded in the course of an hour or two with carbonic acid gas, to the 

 amount of from one-half to two per cent., as has been ascertained 

 both by direct experiment, and by calculation. And there can be no 

 reasonable doubt, that the habitual respiration of such air, in the 

 narrow and noisome dwellings of the poor, or in crowded factories and 

 workshops, has a tendency to produce, both directly and indirectly, 

 much loss of physical and mental vigour, and also to blunt the acute- 

 ness of the moral feelings ; its influence being specially noticed in 

 increasing the predisposition to Epidemic diseases, and in augmenting 

 the fatality of their attacks. 



697. The effects of a simple deficiency of Oxygen in the respired air, 

 are experienced by those who breathe a rarefied atmosphere, such as 

 that which exists on the summits of high mountains. All persons who 

 have made such ascents, have experienced the insufficiency of rarefied 

 air to sustain the degree of respiration required for active exertion. 

 As long as the body remains at rest, no inconvenience is perceived ; 

 but as soon as the muscular system is put into action, the insufficiency 

 of the supply of oxygen is manifested by the feeling of distress and 

 languor ; which becomes so severe, that the individual, if unused to 

 such ascents, is obliged to stop and take breath at every few steps. 

 The necessity for doing so will be easily understood, when it is remem- 

 bered that when the pressure of the atmosphere is reduced to half its 

 usual amount, the bulk of a given weight of air will be twice as great 

 as at the surface of the earth, or the same measure will weigh only 



