1 16 Mr. W. F. Barrett on a Physical 



We have here raised the absorption by the pure gas, but the 

 calculated percentage is still very wide of the mark. Theoreti- 

 cally, nearly 1*6 inch of the pure carbonic acid should give the 

 same absorption as 30 inches of breath from bag No. IV. The 

 discrepancy was evidently not owing to any external cause ; and 

 the only difference was, that in the case of breath the carbonic 

 acid was mixed with air, whilst with the pure gas no such medium 

 was present. Possibly the air might have had an effect upon 

 the carbonic acid somewhat similar to that which it is known 

 to have upon aqueous vapour, by preventing any partial conden- 

 sation of the gas. The following is the result of an experiment 

 made to decide this question : — 



Table IX. — Pure carbonic acid gas and dry air. 



Tension, in Deflection. Absorption 



inches. per 100. 



1-0 29-8 44*4 



1-3 320 47-7 



Tube now filled with dry air . 34*2 53-8 



Total heat 46*1 100-0 



Here carbonic acid at 1*3 inch tension absorbed nearly 47' 7 

 per cent, of the heat from the carbonic oxide flame; but when 

 28 inches of dry air, perfectly inert when alone, were added to 

 the gas in the tube, the absorption was raised to 53*8, or more 

 than 6 per cent. This singular effect is most probably owing to 

 the cause above mentioned, namely, that carbonic acid at small 

 tensions is partially condensed on the polished surface of the 

 interior of the tube : the entrance of dry air removes the film, 

 and consequently throws a larger amount of gas in the path of 

 the rays from the source. Many experiments were made to put 

 this result beyond doubt, and to determine with certainty the 

 absorption by carbonic acid at different tensions when it was 

 mixed with dry air. It was found to be the best mode of experi- 

 ment to admit into the tube a certain quantity, say 20 inches, of 

 dry air first, observing its absorption, if any, and when the 

 needle was at zero, adding a definite amount of pure carbonic 

 acid to the air in the tube. The quantity of gas admitted was 

 accurately found by observing, through a magnifying lens, the 

 depression of the barometer-gauge attached to the air-pump, 

 and which was in communication with the experimental tube. 

 The following Table contains some of the results : — 



