382 THE TRACHEA I. SYSTEM OF THE IMAGO. 



Action of Films on Gases. The escape of carbon dioxide from 

 the surface is possibly complicated by another observed pheno- 

 menon. Graham showed that caoutchouc and other colloid 

 substances have the property of condensing certain gases in 

 their substance, and that these then escape from an exposed 

 surface like a highly volatile liquid. Now, chitin is a colloid 

 substance, and although it is permeable to all gases like a 

 porous membrane, it is possible that it has a special power of 

 absorbing carbon dioxide ; which, from its weight, transfuses 

 slowly when compared with oxygen and nitrogen. The rapid 

 transfer of carbon dioxide observed by Dewitz [159 ] is suggestive 

 of this explanation. 



Transpiration of Nitrogen. - - The greatest difficulty which 

 occurs in the acceptance of the view here advocated is the 

 nitrogen of the air. It would appear at first sight that the 

 nitrogen must accumulate in the tracheal tubes, owing to its 

 slight solubility in the fluids of the body. 



Graham showed that the transpiration of a gas is dependent 

 on its pressure, and that equal weights of various gases under- 

 go transpiration in equal times. Now the tension of a gas in 

 solution is dependent on the tension at which it is absorbed, 

 and Peyron showed that the percentage of nitrogen is as large 

 in the mixed gases abstracted from the insect as in the air ; as 

 the solubility of nitrogen is small, and that of carbon dioxide 

 is large, it follows that the greater part of the nitrogen is in 

 the tracheae in a gaseous condition, whilst the greater part of 

 the carbon dioxide is necessarily in solution in the fluids of 

 the insect. It may therefore be concluded that the nitrogen 

 pressure in the tracheal tubes is at least 90 per cent, of the 

 total air-pressure in those tubes. If I am right in the view 

 that the pressure of the air in the tracheae is from 50 to 

 75 mm. above the atmospheric pressure, taking the lowest of 

 these numbers and adding it to the atmospheric pressure, 

 760 + 50 = 810 mm., the intra-tracheal pressure, and if 90 per 

 cent, is nitrogen pressure, this equals 729 mm., whilst the 

 partial pressure of the nitrogen in the atmosphere is approxi- 

 mately ')OO mm., hence there is a difference of pressure 



