Radiation of Heat by Gases and Vapours. 275 



The proportion here holds good up to a tension of 2 - 5 inches, 

 when the deviation from it commences and gradually augments. 



Though these measurements were made with all possible care, 

 I should like to repeat them. Dense fumes issued from the 

 cylinders of the air-pump on exhausting the tube of this gas, and 

 I am not at present able to state with confidence that a trace of 

 such in a very diffuse form within the tube did not interfere with 

 the purity of the results. 



Table XXII.— Nitrous Oxide. 





Absorption. 





Tension in inches. 



Observed. ( 



Calculated 



0-5 



14-5 



145 



1-0 



23-5 



29-0 



1-5 



30-0 



43-5 



2-0 



35-5 



58-0 



2-5 



41-0 



71-5 



3-0 



45-0 



87-0 



3-5 



47-7 



101-5 



4-0 



49-0 



116-0 



4-5 



51-5 



130-5 



5-0 



54-0 



145-0 



Here the divergence from proportionality makes itself mani- 

 fest from the commencement. 



I promised at the first page of this memoir to allude to the 

 results of Dr. Franz, and I will now do so. With a tube 3 feet 

 long and blackened within, an absorption of 3*54 per cent, by 

 atmospheric air was observed in his experiments. In my expe- 

 riments, however, with a tube 4 feet long and polished within, 

 which makes the distance traversed by the reflected rays more 

 than 4 feet, the absorption is only one-tenth of the above amount. 

 In the experiments of Dr. Franz, carbonic acid appears as a 

 feebler absorber than oxygen. According to my experiments, for 

 small quantities the absorptive power of the former is about 150 

 times that of the latter; and for atmospheric tensions, carbonic 

 acid probably absorbs nearly 100 times as much as oxygen. 



The differences between Dr. Franz and myself admit, perhaps, 

 of the following explanation. His source of heat was an argand 

 lamp, and the ends of his experimental tube were stopped with 

 plates of glass. Now Melloni has shown that fully 61 per cent, 

 of the heat-rays emanating from a Locatelli lamp are absorbed 

 by a plate of glass one-tenth of an inch in thickness. Hence in 

 all probability the greater portion of the rays issuing from the 

 lamp of Dr. Franz was expended in heating the two glass ends 

 of his experimental tube. These ends thus became secondary 

 sources of heat which radiated against his pile. On admitting 

 air into the tube, the partial withdrawal by conduction and con- 



