INTERSTITIAL MOVEMENTS OF SOLIDS, LIQUIDS, GASES. 151 



diffusion of gases, and the condensing action of membranes, _ 



rrn -i Physical prm- 



that we have now to deal. These give us so precise and per- cipies alone re- 



spicuous an explanation of the act of breathing that it is 



needless to look beyond them ; yet on that act depend the respiratory en- 



highest operations of life. In this particular the Scriptures glne 



have summed up the deductions of modern physiology in a single line 



no metaphorical expression, but the simple assertion of a truth : He 



" breathed into his nostrils the breath of life, and man became a living 



soul." 



Of the physical principles now to be dealt with, it is unnecessary to 

 say any thing respecting the pressure of the atmosphere, since that is 

 well understood; but not so with the phenomena of the diffusion of gas- 

 es, and the condensing action of membranes. Though these are subjects 

 which have been particularly examined by American physicians, the facts 

 they have elicited are little known abroad. For example, the error of 

 Valentin's statement respecting the diffusion exchanges of carbonic acid 

 and oxygen, and the uselessness of the elaborate discussions which have 

 originated therefrom, would at once have been recognized, had attention 

 been directed to the facts developed here almost twenty years ago. 

 , Interstitial motions are exhibited by solids, liquids, and gases. I 

 have had occasion to examine Roman silver coins, from the interstitial 

 interior of which the copper originally present had made its ^Ji^widK^ 

 way out to the surface, forming the greenish incrustation uids. 

 known as patina by antiquarians, the silver being left almost pure. In 

 speaking of absorption by the blood-vessels in Chapter VI., we had oc- 

 casion to dwell upon the same propensity as shown by liquids, the en- 

 dosmosis of Dutrochet being an example of it. The ready mobility of 

 this group of bodies, arising from their diminished cohesion, greatly pro- 

 motes these effects. Mr. Boyle collected a number of cases of solid move- 

 ments in his tract on the languid motions of bodies. 



Gases and vapors, by reason of their total want of cohesion, present the 

 most striking examples of these effects. Their propensity to intermin- 

 gle with each other is manifested, even though they be obliged to pass 

 through crevices or winding passages. One of the first instances to which 

 attention was directed occurred under the observation of Dr. Priestley's ob- 

 Priestley, who found, on passing steam through an earthen 

 tube placed in a furnace, that air would be delivered at the of gases. 

 farther end. For some time he supposed that this experiment demon- 

 strated the conversion of water into air by a great heat f but eventually 

 traced it to its proper cause the escape of the steam outward through 

 the pores of the earthen tube, and the intrusion in the opposite direc- 

 tion of air from the furnace. This singular experiment may be well 

 shown by attempting to pass steam through a red-hot tobacco-pipe, the 



