IS MM. Lecher and Pern tor on the Absorption 



gas-holder, and this moist air then passed through several 

 wash-bottles into the experimental space, but with the same 

 negative result. This result was so much the more surprising, 

 since all experiments show absorption of the heat-rays of the 

 sun by the atmosphere, which, since it cannot be ascribed to 

 the pure air, has universally been put down to the account of 

 the aqueous vapour. 



In opposition to this result is that obtained by Tyndall, 

 who found an absorption of 4-6 per cent. * with a source of 

 heat at 100° C, as also at higher temperatures. 



It was shown above that vapour-adhesion must take place 

 on the rock-salt and on the polished inner walls of the tube. 

 To this we have nothing more of importance to add ; only the 

 remark may be allowed that Tyndall himself does not con- 

 sider his experiments in air, without a tube, decisive, and 

 that the reflectors of the thermopile were allowed to remain, 

 thus partially replacing the tube — an objection which applies 

 also to Hoorweg'sf work. 



We would express in the form of a question a thought 

 which has frequently occurred to us during the study of this 

 subject. Why did not Tyndall take the simplest way of 

 setting aside the objections of Magnus, and simply use a 

 blackened tube provided with diaphragms instead of the 

 polished one ? Why did he content himself with making one 

 experiment (in a case so readily open to objection) with a half- 

 blackened tube, and then take his stand upon the proportion- 

 ality of the action ? The matter seems so simple that we have 

 never been able to understand why Tyndall did not experiment 

 with blackened tubes. 



A statement which w T ill possibly be quite convincing we 

 leave till further on. 



In what follows we give the results of our experiments on 

 gases and vapours. 



As far as the first are concerned, our numbers differs very 

 little from those of Tyndall ; whilst with vapours we have 

 almost always obtained different values. Many of them were 



* Loc. cit. pp. 133, 134. 



t Loc. cit. p. 131. The experiments repeated by Frankland, at Tyndall's 

 request, lead to the same result (loc. cit. p. 183). For air saturated with, 

 aqueous vapour at a temperature of about 12° C. lie gives 5| per cent. 

 Iloorweg (rogg. Ann. civ.) found also an absorption for aqueous vapour, 

 though only about half as much as Tyndall gives. The method of allow- 

 ing air saturated with aqueous vapour to rise freely does not appear to 

 ua to be free from objection. Would not invisible globules of water form 

 there very easily P and would the walls of the reflectors and of the ther- 

 mopile actually remain unaffected ? 



