Prof. Magnus on the Conduction of Heat by Gases. 511 



those gases which have a greater coefficient of expansion must 

 be greater than in the rest ; for example, in carbonic acid more 

 than in hydrogen. As this is not the case, it must either be as- 

 sumed that the friction of the gaseous particles against each 

 other is so great that the influence of the greater expansion is 

 neutralized by it, which will with difficulty be admitted, or it 

 must be assumed that gases by contact with a hot body 

 become heated to a different extent. Such a difference in the 

 degree of heat would take place if the gases had different capa- 

 cities for heat ; but as the specific heats of hydrogen and atmo- 

 spheric air are the same, there remains no other explanation for 

 the more rapid cooling in hydrogen than that this gas can 

 transmit heat from particle to particle, in other words, can con- 

 duct it, and that it possesses this property in a higher degree 

 than other gases. Its low density appeared to be in disaccord- 

 ance with this idea, and it appeared necessary to decide by ex- 

 periments how far it is founded. 



The impulse to these experiments was given by a repetition 

 of Mr. Grove's interesting observation, according to which a 

 platinum wire is less strongly heated when surrounded by 

 hydrogen than by atmospheric air, or another gas. In this 

 repetition it was found that hydrogen exerted its preventive 

 action even when a layer only 0*5 millirn. thick surrounded 

 the wire, and it was the same whether the tube containing it 

 was in a horizontal or vertical position. In such a narrow tube, 

 especially when it is horizontal, currents can scarcely occur; 

 and when there are none, there remains no other explanation 

 than that hydrogen conducts heat better than other bodies. 



The simplest mode of ascertaining whether a gas conducts 

 heat, consists in warming it from above, and observing the 

 action on a thermometer placed within. It might be objected 

 to this method, that, even with heating from above, currents in 

 the gas might be formed, and that thereby the temperature in- 

 dicated by the thermometer in various gases might be different 

 without any difference in conductibility. 



There is one method of testing this objection. For if, in fact, 

 a gas can conduct heat, the temperature assumed by a ther- 

 mometer in a space heated from above must be lower when the 

 conducting substance is wanting than when it is present ; that 

 is, it must be lower in vacuo than in a space filled with air. 



In order to ascertain whether this was the case, a glass appa- 

 ratus was used, in which a thermometer, observable from with- 

 out, was firmly fixed. It could be filled with different gases, 

 and these could be variously dilated. The upper part of this 

 apparatus was maintained at the same temperature, namely that 

 of boiling water, and the temperature was observed which a 



