68 MATERIAL COMPOSITION OF MAN. 



them, as it were, without obstacle. If we take a bladder, and fill it 

 with pure hydrogen, and afterwards leave it in contact with atmospheric 

 air, in a very short time the hydrogen will have lost its purity, and 

 be mixed with the atmospheric air, which has penetrated the bladder. 

 This phenomenon is more rapid in proportion as the membrane is thin- 

 ner and less dense. It presides over one of the most important acts of 

 life respiration ; and continues after death." 



Dr. Mitchell is the first individual, who directed his observation to 

 the relative penetrativeness of different gases. This he was enabled 

 to discriminate by the following satisfactory experiment, which we 

 give in his own words: " Having constructed a syphon of glass, with 

 one limb three inches long, and the other ten or twelve inches, the 

 open end of the short leg was enlarged and formed into the shape of 

 a funnel, over which, finally, was firmly tied a piece of thin gum 

 elastic. By inverting this syphon, and pouring into its longer limb 

 some clear mercury, a portion of common air was shut up in the short 

 leg, and was in communication with the membrane. Over this end, in 

 the mercurial trough, was placed the vessel containing the gas to be 

 tried, and its velocity of penetration measured by the time occupied in 

 elevating to a given degree the mercurial column in the other limb. 

 Having thus compared the gases with common air, and subsequently 

 by the same instrument, and in bottles with each other, I was able 

 to arrange the following gases according to their relative facility of 

 transmission, beginning with the most powerful: ammonia, sulphu- 

 retted hydrogen, cyanogen, carbonic acid, nitrous oxide, arseniuretted 

 hydrogen, olefiant gas, hydrogen, oxygen, carbonic oxide, and nitro- 

 gen." 



He found that ammonia transmitted in one minute as much in volume 

 as sulphuretted hydrogen did in two minutes and a half; cyanogen, 

 in three minutes and a quarter; carbonic acid, in five minutes and a 

 half ; nitrous oxide, in six minutes and a half ; arseniuretted hydrogen, 

 in twenty-seven minutes and a half ; olefiant gas, in twenty-eight 

 minutes; hydrogen, in thirty-seven minutes and a half; oxygen, in one 

 hour and fifty-three minutes ; and carbonic oxide, in two hours and 

 forty minutes. It was found, too, that up to a pressure of sixty-three 

 inches of mercury, equal to more than the weight of two atmospheres, 

 the penetrative action was capable of conveying the gases the sub- 

 jects of the experiment into the short leg through the gum elastic 

 membrane. Hence, the degree of force exerted in the penetration is 

 considerable. 



The experiments were all repeated with animal membranes, such as 

 dried bladder and gold-beater's skin, moistened so as to resemble the 

 natural state. The same results, and in the same order, followed as 

 with the gum elastic. The more fresh the membrane, the more speedy 

 and extensive was the effect; and in living animals the transmission 

 was very rapid. 



To these experiments there will be frequent occasion to refer in the 

 course of this work. 1 



1 See, connected with this subject, the ingenious papers by Dr. Robert E. Rogers, and Dr. 

 Draper, the former in the American Journal of the Medical Sciences , May, 183(3, p. 13; 



