382 THE TRACHEAL 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 trachez 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 go 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 tracheze 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 go per 
cent. is nitrogen pressure, this equals 729 mm., whilst the 
partial pressure of the nitrogen in the atmosphere is approxi- 
mately 600 mm., hence there is a difference of pressure 
