160 ANNUAL OF SCIENTIFIC DISCOVERY. 







the rest ; for example, in carbonic acid more than in hydrogen. As 

 this is not the case, it must either be assumed 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 capacities 

 for heat ; but as the specific heats of hydrogen and atmospheric air are 

 the same, there remains no other explanation for the more rapid cool- 

 ing in hydrogen, than that this gas can transmit heat from particle to 

 particle, in other words, can conduct it, and that it possesses this prop- 

 erty in a higher degree than other gases. Its low density appeared 

 to be in disaccordance with this idea, and it appeared necessary to 

 decide by experiments how far it is founded. 



The impulse to the^e 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 atmos- 

 pheric air, or another gas. In this repetition it was found that hy- 

 drogen exerted its preventive action even when a layer only 0.5 mil. 

 thick surrounded the wire, and it was the same whether the tube con- 

 taining 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 hvdrogen conducts heat better than other bodies. 



/ o 



The simplest mode of ascertaining whether a gas conducts heat 

 consists in warming it from above, and observing the action on a ther- 

 mometer 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 indicated by the thermometer in 

 various gases might be different without any difference in conducti- 

 bility. 



There is one method of testing this objection ; for if, in fact, a 

 gas can conduct heat, the temperature assumed by a thermometer in 

 a space heated from above must be lower when the conducting sub- 

 stance is wanting than when it is present ; that is, it must be lower in 

 vacua than in a space filled with air. 



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

 was used, in which a thermometer, observable from without, was 

 firmly fixed. It could be filled with different gases, and these could 

 be variously dilated. The upper part of this apparatus was main- 

 tained at the same temperature, namely, that of boiling water, and 

 the temperature was observed which a thermometer introduced into 

 the interior ultimately assumed. Of course the experiments with this 

 apparatus were not made without numerous precautions ; it was more 

 particularly necessary that the whole apparatus should be always 

 under the same conditions, so as to give off the heat imparted to it 

 always in the same manner. For this it was necessary that the space 

 surrounding it should always be at the same temperature. In these 

 experiments, the temperature of the surrounding space was 15. 



In this way the following results were obtained : 1. The temper- 

 ature which a thermometer ultimately assumes in a space heated from 

 above differs when tin's space is filled with different gases. 2. In hy- 



