277 



CHAPTEE IX 



CONDUCTION OF HEAT 



1O3. Low Conductivity of Gases 



ONLY very little was known respecting the conduction of 

 heat in gases before the development of the kinetic theory 

 of heat. The experiments made by Andrews, 1 Magnus, 2 

 and Peclet 3 show nothing more than that heat is propa- 

 gated in gaseous media not only by radiation, but also by 

 conduction as in solid and liquid bodies ; but the experi- 

 ments gave no measurements of the conductivity of gases, 

 and only showed that it is very much less than the con- 

 ductivity of solids and liquids. Among the gases hydrogen 

 is distinguished, as was long ago known to Priestley 4 

 and Ac hard, 5 by so considerably greater a conductivity that 

 Magnus thought it might be compared with that of 

 metals; however, later observations have shown it to be 

 something like 1,000 times less than that. 



The vapours also of liquids, just like the real gases, have 

 only a small capacity for conducting heat. The most 

 striking proof of this fact is furnished byLeidenfrost's 6 

 experiment with a drop of water on a hot plate, as it shows 

 that the layer of vapour which is formed between the drop 

 and the plate prevents the passage of heat to such an extent 

 that the water does not reach boiling-point. 



1 Proc. Irish Acad. 1841, i. p. 465 ; Berzelius' Jahresb. xxii. p. 24. 



2 Pogg. Ann. 1861, cxii. p. 497. 



3 Traite de la Chaleur, 3rd ed. 1861, iii. p. 418. 



4 Experiments and Observations relating to Various Branches of Natural 

 Philosophy, Birmingham 1781, i. sect. 33, 5, p. 378. 



5 Mem. de I' Acad. de Berlin, Annee 1783, p. 84, Berlin 1785. 



6 Leidenfrost, De aquce communis nonnullis qualitatibus tractatus, 51, 

 Duisburgi 1756. Opuscula phys.-chem. et med. iii., Duisburgi 1797. 



