On the Structure and Functions of the Organs of Respiration. 219 



legists. The discoveries of Dutrochet, however, on the endosmose and 

 exosmose of fluids, and tlie parallel experiments of Dr. Stevens, Dr. Mit- 

 chell, and Mr. Graham, on the mutual penetration of the gases, and their 

 transmission through membranes, have dissipated the obscurity which pre- 

 viously enveloped the subject, by showing that the process is entirely ex- 

 plicable on principles which we can readily comprehend. 



All gases, of different densities, which are not disposed to unite chemi- 

 cally with one another, have a strong tendency to mutual admixture. Thus, 

 if a vessel be partly filled with hydrogen, and partly with carbonic acid, the 

 latter, which is twenty-two times heavier than the former, will not remain 

 at the bottom, but the two gases will be found in a short time to have 

 uniformly and equably mixed : and it is on this principle, that the consti- 

 tution of the atmosphere is found to be every where the same, although 

 the gases which compose it are of very different specific gravities. So 

 strong is the tendency to admixture, that it will take place when a mem- 

 brane or other porous medium is interposed between the gases ; as, for 

 instance, when a bladder of hydrogen is placed in an atmosphere of carbonic 

 acid : a certain quantity of hydrogen will pass out of the bladder, and at 

 the same time carbonic acid will enter j but the relative quantities of the 

 two will vary according to several circumstances, of which the comparative 

 densities of the gases, and the facility with which the membrane is per- 

 meated by each, are the principal. When plaster of Paris is employed as 

 the medium of diffusion, the exchange will take place under the sole in- 

 fluence of the former : and a general law has been ascertained, which ap- 

 plies to all instances ; that the replacing or mutual diffusion of volumes of 

 different gases varies inverseli/ as the square roots of their densities. Thus, 

 if a tube, closed at one end with a plug of plaster of Paris, be filled with 

 hydrogen, the gas will soon be entirely removed, and will be replaced by 

 something less than one fourth of its bulk of atmospheric air, the density 

 of hydrogen being about -^^ that of the atmosphere. 



In the same mannner, if a fluid be charged with any gas which it will 

 absorb, (as, for example, water with carbonic acid,) it will speedily part 

 with it when exposed to the attracting influence of another gas, such as 

 atmospheric air j and the more different the densities of the two gases, the 

 more rapidly, and with more force, will this effect take place. As in the 

 former case, this attraction will go on, with little interruption, through a 

 porous membrane ; and part of the exterior gas will be absorbed by the 

 fluid, in place of that which has been removed. 



These simple facts will be found a key to the explanation of the chemical 

 changes which take place both in the circulating fluid and the respired 

 air. 



We do not yet know enough of the physiological anatomy of vegetables, 

 to be able to define with certainty the uses of each different part of the 

 system j and indeed there is often much difficulty in specifying the exact 



