152 PEINCIPLES OF CHEMISTRY 



about chemical processes in them, and chiefly extracting carbon from 

 them in the form of carbonic anhydride, the greater part of which 

 passes into the blood, is dissolved by it, and is thrown off by the lungs 

 during the absorption of the oxygen. Thus, in the process of respiration 

 carbonic anhydride (and water) is given off, and the oxygen of the air 

 absorbed, by which means the blood is changed from a dark-red 

 venous to a bright-red arterial blood. The cessation of this process causes 

 death, because then all those chemical processes, and the consequent 

 heat and work which the oxygen introduced into the system brought 

 about, ceases. For this reason suffocation and death ensue in a vacuum, 

 or in a gas which does not contain free oxygen (which does not support 

 combustion). If an animal be placed in an atmosphere of free oxygen, 

 then at first its movements are very active and a general invigoration is 

 remarked, but a reaction soon sets in, and perhaps death may ensue. 

 The oxygen of the air, when it enters the lungs, is diluted with four 

 volumes of nitrogen, which is not absorbed into the system, and there- 

 fore the blood absorbs but a small quantity of oxygen from the air, 

 whilst in an atmosphere of pure oxygen a large quantity of oxygen 

 would be absorbed, which would produce a very rapid change of all parts 

 of the organism, and destroy it. From what has been said, it will be 

 understood that oxygen may be employed in respiration, at least for a 

 limited time, when the respiratory organs suffer under certain forms of 

 suffocation and impediment to breathing. 2 



The combustion of organic substances that is, substances which 

 make up the composition of plants and animals- proceeds in the 

 same manner as the combustion of many inorganic substances, such as 

 sulphur, phosphorus, iron, &c., from the combination of these sub- 

 stances with oxygen, as was described in the Introduction. The de- 

 composition, rotting, and similar transformations of substances, which 



2 It is evident that the partial pressure (see Chap. II.) acts in respiration. The researches 

 of Paul Bert showed this with particular clearness. Under a pressure of one-fifth of an at- 

 mosphere consisting of oxygen only, animals and human beings remain under the ordinary 

 conditions of the partial pressure of oxygen, but organisms cannot support air rarefied to one- 

 fifth, for then the partial pressure of the oxygen falls to one-twenty-fifth of an atmosphere. 

 Even under a pressure of one-third of an atmosphere the regular life of human beings is im- 

 possible, by reason of the impossibility of respiration (of the decrease of solubility of oxygen 

 in the blood), owing to the small partial pressure of the oxygen, and not from the mechani- 

 cal effect of the decrease of pressure. Paul Bert illustrated all this by many experiments, 

 some of which he conducted on himself. This explains, among other things, the discom- 

 fort felt in the ascent of high mountains or in balloons when the height reached exceeds 

 eight kilometres, and at pressures below 250 mm. (Chap, II. note 23). It is evident that 

 an artificial atmosphere has to be employed in the ascent to great heights, just as in sub- 

 marine work. The cure by compressed and rarefied air which is practised in certain ill- 

 nesses is based partly on the mechanical action of the change of pressure, and partly on 

 the alteration in the partial pressure of the respired oxygen. 



