156 GENERAL PRINCIPLE OF DIFFUSION. 



quickly became brown. So sulphureted hydrogen can pass through a 

 sheet of India-rubber and diffuse into an atmosphere of oxygen, hydro- 

 gen, and atmospheric air beyond, though it is resisted by a pressure equal 

 to that of 800 feet of water. 



The method of condensation here employed, because of its freedom from 

 mechanical concussions, enabled me to continue these researches up to 

 pressures of 50 atmospheres without leakage, in comparatively slender 

 tubes, and even under these circumstances gaseous diffusion seemed to 

 take place without any restraint. 



It would lead me too far from my present object to pursue the con- 

 General facts sideration f these facts, and I must therefore be content to 

 connected with refer the reader to the memoirs in which they have been spe- 

 diffusion. cia j lv di scusse( k* i t i s sufficient to understand, 1st. That 

 gases simply exposed to each other inter-diffuse with great rapidity, and 

 at a rate inversely proportioned to the square root of their densities ; 2d. 

 That the same takes place through stucco plugs, or diaphragms with open 

 pores ; 3d. That a gas dissolved in a liquid, or held in a condensed state 

 by a solid mass, will exchange by inter-diffusion with any atmosphere 

 to which it may be exposed, in these cases the liquid or the solid mass 

 becoming a source of force ; 4th. That through a liquid, which, of course, 

 has no pores, gases arranged on its opposite sides will diffuse, but their 

 rate is no longer expressed by Graham's law ; 5th. That a liquid hold- 

 ing a gas in solution permits it to diffuse with another gas held by an- 

 other liquid in solution. 



On the first of these principles, the fresh air of the bronchial tubes ex- 

 changes with the respired air of the pulmonary cells, the case being that 

 of a gas exposed to a gas. On the third of these principles, arterializa- 

 tion of the blood takes place, the case being that of a dissolved gas ex- 

 changing with a free gas ; and on the fifth of these principles, aquatic or 

 gill respiration depends, the case being that of a dissolved gas exchang- 

 ing with another dissolved gas. 



Under its simplest aspect, the act of breathing consists in the elimina- 

 Various forms ^ on ^ car b oiu ' c acid from the system, and the introduction 

 of respiratory of oxygen. The manner in which the respiratory surface 

 itself from the former, and secures new supplies of the 



mechanism. 



latter, differs very greatly. In the lower orders which lead an aquatic 

 life, currents are established in the water by the aid of ciliary motion, and 

 by these the necessary changes are made. In others, in which respira- 

 tion is conducted by the skin, incessant locomotion is relied on ; and 

 again, in others, the water is drawn into the stomach and intestinal canal, 

 and every part bathed with the aerating medium. 



In insects, the type of carrying air to the blood is developed to the ut- 

 * American Journal of Medical Sciences, May, 1838. 



