MM. Lecher and Pernter on the Absorption 



the contrary, finds that a layer of air 275 millims. in thick- 

 ness is Bumciont to absorb 11 percent.; and, lastly, Buff* 

 believes he lias observed an absorption of 40 per cent, by a layer 

 only 45 millims. in thickness. It is remarkable that these 

 results are in inverse proportion to the length of the absorb- 

 ing layer, and so illustrate only the more strikingly the inex- 

 plicable difference in the results of such excellent experi- 

 menters. 



The uncertainty which these results left on the one hand, 

 and on the other the indubitable absorption of heat-rays in 

 the atmosphere, especially as shown by the pyrheliometric in- 

 vestigations of Viollef and CrovaJ, induced us to undertake 

 the present investigation. If it should lead to the conclusion 

 that the question can hardly be decided by absolute measure- 

 ments in the laboratory, yet the results obtained may at least 

 furnish definite starting-points for its decision. Further, 

 the numbers given by Tyndall for the vapours have been sub- 

 jected to accurate investigation; so that some new points of 

 view present themselves in respect to physics. 



When, in what follows, we give numbers which for the 

 most part deviate from those previously found, we are well 

 aware of the difference, and wish at the outset to emphasize the 

 fact that the values in question, in spite of their deceptive 

 coincidence with each other, are to be regarded as only some- 

 what rough approximations. But we shall show immediately 

 that all numbers hitherto given, including those of Tyndall 

 which relate to vapours, are vitiated by important errors 

 resulting from the mode of observation employed. 



Remarks on Methods previously employed. 



There are two essentially different methods which may be 

 employed : the thermopile and the source of heat may be 

 placed in the same chamber in which the gas or vapour under 

 examination is placed ; or one, or both, may be outside of the 

 space filled with gas. The first method has been employed 

 especially by Magnus §, and then by Garibaldi|| and by 

 BufHI. 



* Phil. Mag. [5] iv. ; Pogg. Ann. clvii. 



t Camp. Itend. t. lxxxii. 1808, 1. 



\ Manures de Tmtmsitt des radiations solaires, par M. A. Crova. Paris, 

 1870 (Gauthier-Tillars). 



§ Pogg. Ann. cxii. 



|| 11 nuovo Oimento, ser. 2, t. iii. This work was accessible to us only 

 by means of the reports in the Naturforscfier, Jahrg. iv. 1871, Nr. 33, and 

 in Fori. schrUte der Fhymk, 1871. 



11 Phil. Mag. [5] iv. 



