944 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



of oxygen is evolved on the positive pole and two volumes of hydrogen on the 

 negative. 



(2) Through the action of zinc on sulphuric acid, 1 



(3) Through putrefaction (by which is understood the change effected in 

 organic matter through certain lower organisms, bacteria) hydrogen is liberated 

 in the intestinal canal from proteid matter, and especially from the fermenta- 

 tion of carbohydrates : 



C 6 H 12 6 = C 4 H 8 2 + 2C0 2 + 2H 2 . 



Sugar. Butyric acid. 



In putrefaction in the presence of oxygen the hydrogen formed immediately 

 unites with oxygen, producing water ; hence, notwithstanding the enormous 

 amount of putrefaction in the world, there is no accumulation of hydrogen 

 in the atmosphere. 



Both bacteria and an enzyme can liberate hydrogen by acting on calcium formate, 



Ca (CH0 2 ) 2 + H 2 = CaC0 3 + C0 2 + 2H 2 , 



and this same reaction may be brought about by the action of metallic iridium, rhodium, 

 or ruthenium on formic acid. An enzyme is a substance probably of proteid nature capa- 

 ble of producing change in other substances without itself undergoing apparent change 

 (example, pepsin). Bunge 2 calls attention to the fact that tbe above reaction may be brought 

 about by living cells (bacteria), by an organic substance (enzyme), and by an inorganic 

 metal. This similarity of action between organized and unorganized material, between 

 living and dead substances, is shown more and more conspicuously as science advances. 



Properties. Hydrogen burns in the air, forming water, and if two volumes 

 of hydrogen and one of oxygen be ignited, they unite with a loud explosion. 

 Hydrogen will not support respiration, but, mixed with oxygen, may be 

 respired, probably being dissolved in the fluids of the body as an inert gas, 

 without effect upon the organism. Hydrogen may pass through the intes- 

 tinal tissues into the blood-vessels, according to the laws of diffusion, in ex- 

 change for some other gas, and may then be given off in the lungs. Nascent 

 hydrogen that is to say, hydrogen at the moment of generation is a powerful 

 reducing agent, uniting readily with oxygen (see p. 952). 



OXYGEN, O = 16. 



Oxygen is found free in the atmosphere to the amount of about 21 per 

 cent, by volume, and is found dissolved in water and chemically combined in 

 arterial blood. It is swallowed with the food and may be present in the stom- 

 ach, but it entirely disappears in the intestinal canal, being absorbed by respir- 

 atory exchange through the mucous membrane. It occurs chemically com- 

 bined with metals so that it forms one-half the weight of the earth's crust ; 

 it likewise occurs combined in water and in most of the materials forming 

 animal and vegetable organisms. It is found in the blood in loose chemical 



1 It is not within the scope of this work to give more than typical methods of laboratory 

 preparation. For greater detail the reader is referred to works on general chemistry. 



2 Physiolngische Chemie, 2d ed., 1889, p. 167. 



