surrounding Media on Voltaic Ignition. 123 



As far as ordinary ignition is concerned, hydrogen has been 

 shown by the experiments of Leslie and Davy to produce a 

 more rapid cooling effect than air ; and the above experiment 

 having shown that it does not alter or convert into any other 

 force the actual amount of heat given off, my next step was to 

 inquire whether this rapidity of cooling effectof the hydrogen 

 would account for the effects observed with voltaic ignition. 

 Although the two classes of effects were apparently very dif- 

 ferent, it might be that the improved power of conduction 

 arising from the rapid cooling effect of the hydrogen might, 

 by enabling the current to pass more readily, carry off the 

 force in the form of electricity, which if the wire offered more 

 resistance (as it would when more highly ignited) would be 

 developed in the form of heat. By employing the same me- 

 dium, but impeding the circulation of the heated currents in 

 one case, while their circulation was free in the other, some 

 light might be expected to be thrown on the inverse relation 

 of the conducting power to the heat developed. The follow- 

 ing experiment was therefore tried. 



In the apparatus represented in fig. 1, tube A was uncorked, 

 so as to allow free passage for the water, while tube B was 

 filled up w^ith fine sand soaked with water, and then corked 

 at both ends ; the current was passed and the following was 

 the result. In the vessel containing tube A, the thermometer 

 rose in five minutes from 52° to 60°, and in that containing 

 tube B from 52° to 60° also; during a second five minutes, 

 the thermometer rose in the vessel containing A from 60° to 

 67°, and in the vessel containing B from 60° to 67° also. 



I tried another analogous experiment: a coil of platinum 

 wire was placed in a very narrow glass tube one-sixth of an 

 inch diameter; this was hermetically sealed at one end, and 

 the other drawn into a very narrow aperture, little more than 

 sufficient to allow the platinum wire to pass, and filled with 

 water (it was necessary to leave a small aperture to prevent 

 the bursting of the tube by the expansion of the heated water); 

 in the other vessel a similar coil of platinum wire was placed, 

 but without any glass tube at all. The circuit having been 

 completed as before, the thermometer rose in five minutes — 



In the water without the tube, from . 60° to 87°. 

 In the water containing the tube, from 60° to 86°. 



Here the difference, slight as it was, was against what theory 

 would have led one to anticipate; the exact equality however 

 of the previous experiment, and the close approximation of 

 the results in this one, afford no conclusive information as to 

 the point under consideration, though the negative result 



r^v 



