296 PHYSIOLOGY AT .THE FARM. 



venous blood, and being conveyed to the lungs, is there 

 discharged along with what may be formed in the vessels of 

 the lung itself during the reception of new oxygen from the 

 inspired air. 



Hydrogen. Hydrogen is the lightest known substance. It 

 is sixteen times lighter than oxygen, fourteen times lighter 

 than nitrogen, and twenty-two times lighter than carbonic - 

 acid gas. It was once thought that owing to its great levity 

 it might form the highest layer of the atmosphere, and this 

 idea is not wholly incompatible with the view adopted at 

 present as to the mode in which the constituent gases of the 

 atmosphere exist diffused through one another. For if hydro- 

 gen exist in the atmosphere ^and some facts seem to show that 

 it does then it must extend from the surface of the earth 

 gradually diminishing in density, according to the law affect- 

 ing the whole atmosphere, till it reach its utmost limit. But 

 as the relative heights of the several columns of the atmo- 

 spheric gases must be in the inverse proportion of the density 

 of each that is, in the direct proportion of the levity of each 

 the idea referred to is so far correct that the column of car- 

 bonic-acid gas, the most dense of the whole, must be the 

 shortest ; so that the hydrogen must be highest as the lightest, 

 and therefore it will in fact be the sole gas which crowns the 

 summit of the atmosphere. The great source of hydrogen is 

 the waters of the globe, of which it forms one ninth part by 

 weight. It does not exist in the rocks of the crust of the 

 earth, unless in so far as they contain water. Combined with 

 nitrogen, it is present in ammonia. It constitutes about one 

 sixteenth of the whole weight of the tissue in wood, and nearly 

 the same in starch and sugar. Of dried muscular flesh it 

 makes up about one thirteenth by weight. Such, then, are the 

 proportions in which the hydrogen of water contributes to 

 the substance of vegetable and animal tissues. The hydrogen 



