4 MM. Lecher and Perntor on the Absorption 



Bame rays in almost exactly equal proportions. The diather- 

 mancy of rock-salt has always been examined in air, and then 

 the diathermancy of air between rock-salt plates. This 

 objection is completely justified in the employment of Tyn- 

 dall's method ; and it is necessary that the diathermancy of 

 rock-salt should be examined in vacuo. But our experiments 

 made without rock-salt plates have shown that Tyndall's 

 numbers, so far as they relate to air, are perfectly correct*. 



The employment of rock-salt plates may therefore be allowed 

 in experiments with dry gases, since the percentage of rays 

 absorbed does not appear to be materially influenced by the 

 imperfect diathermancy of the rock-salt. 



The case, however, is altogether different with vapours. 

 Tyndall's numerous replies to the objection made by Magnus, 

 that on the walls of the polished tube and, on the inner sur- 

 face's of the rock-salt plates vapour-adhesion must take place, 

 are by no means convincing. 



We wish from Magnus's deductions from particular expe- 

 riments, according to Tyndall's own data, to collect a convin- 

 cing proof of the magnitude of vapour-adhesion. Tyndall 

 has remarked that it may not be a matter of indifference 

 whether the interior of the tube be polished or not. He 

 recognized that exact investigation alone can set at rest the 

 doubt whether the results will remain in the same proportion 

 amongst themselves if the tube be blackened instead of being 

 polished. It would have been the safest to make similar 

 experiments with similar tubes, in the one case polished and 

 in the other blackened, and to take special care that all other 

 conditions remained unaltered. But Tyndall blackened only 

 the half of the tube, and, in the experiment with this last, 

 employed only a vapour-pressure of 0*3 inch, whilst in the 

 experiments with the polished tube 4 feet long the vapour- 

 pressure was 0*5 inch. 



With such an inequality in the conditions, it can be only a 

 chance that the numbers given in Tyndall's table for the 'ab- 

 sorption of the vapours examined are in the same proportion 

 lor the blackened and for the polished tube. Tyndall finds 

 that it is only necessary to multiply the numbers found for 

 the blackened tube by 4*5 in order to obtain the numbers for 

 the polished tube. 



Moreover the numbers given do not agree so very well 

 together. Table XVULf contains nine substances, for which 

 the numbers are as follows: — 



* Moreover Tyndall has recently directly refuted Buff's objection (Proc. 

 Roy. Soc. voj. xxx. p. 19). This paper is known to us only by the report 



lis and all following translations are as literal as 



in Naturforscher, Jahrg. xiii. No.16. 

 t Loc. cit. p. oo. Thi 



