Vapour in Experiments on the Absorption of Heat. 417 



I obtained with tubes of tinplate, which I frequently used, the 

 same results as with brass ones. 



I will merely remark, further, that the actions mentioned 

 were not only observed with obscure heat, but also by using 

 sources of heat of very high temperature. For if the cube placed 

 before the observing-tube was replaced by a platinum disk which 

 was kept at nearly a white heat by a Bunsen's burner, the phe- 

 nomena observed were almost the same. 



How great is the influence of the inner surface of the tube is 

 evident also from the following observations. When a metal tube 

 was very strongly coated inside with lampblack, the action of air 

 blown in was exactly the opposite of that which occurred in the 

 polished tube. In that case there was an increase of heating effect 

 by forcing in moist air, and a decrease by dry air. The same 

 result was obtained when a tube was coated with cotton velvet 

 instead of with lampblack. The total quantity of heat which passes 

 through such a tube is altogether small ; hence the increase on 

 forcing in moist air is inconsiderable ; but it does exist. I do not 

 venture to give it in percentages, because it depends on acci- 

 dental circumstances, more even than the cooling in polished 

 tubes ; but it amounted to at least 1 per cent. 



With a cardboard tube quite smooth on the inside, there was 

 neither heating nor cooling. But when carbonic acid was admitted 

 into the tube, a cooling occurred which amounted to 7 per cent, 

 of the heat which reached the pile. With another cardboard 

 tube, however, which was lined on the inside with coarse paper, 

 forcing in moist air produced a heating effect, as in the case of 

 the velvet-lined tube. But when the same cardboard tube was 

 lined internally with thin tinfoil, the action was quite the reverse 

 — that is, just as with metal tubes. 



If accordingly there is no doubt that the inner surface of the 

 tube occasions heating or cooling when moist air is forced in, the 

 question arises how this action is produced. 



I have formerly observed* that solid bodies, both metallic 

 and non-metallic, attract aqueous vapours from the surrounding 

 air and condense them on their surface. These condensed va- 

 pours cannot, it is true, be seen ; but the heating which occurs 

 if a solid is placed in contact with moist air of the same tem- 

 perature, and the corresponding cooling by dry air, admit of no 

 other explanation. Such a condensation of the aqueous vapours 

 obviously takes place on the insides of the tubes on forcing in 

 moist air; and it may be shown that the results observed by 

 Professor Wild, and before him by Professor Tyndall, depend on 

 this. 



I will first of all mention how I have convinced myself that 

 * Poggendorff's Annalen, vol. €xxi. p. 174. 



Phil. Mag. S. 4. Vol. 33. No. 225. June 1867. 2 E 



