336 EXPERIMENTAL INVESTIGATIONS. 



ignition which will occur to those who duly consider the sub- 

 ject, promise, I venture to believe, new methods and powers 

 of investigating the molecular constitution of matter, and 

 will, I trust, lead to novel and important results. 



ON THE EFFECT OF SURROUNDING MEDIA ON 

 VOLTAIC IGNITION. 



Phil. Trans. R. S. Received August 10. Read December 14, 1848. 



IN the 'Philosophical Magazine' for -December 1845 I 

 pointed out a striking difference between the heat generated 

 in a platinum wire by a voltaic current, according as the wire 

 is immersed in atmospheric air or in hydrogen gas, and in the 

 Bakerian Lecture for 1847 I have given some farther experi- 

 ments on this subject, in which the wire was ignited in atmo- 

 spheres of various gases, while a voltameter enclosed in the 

 circuit yielded an amount of gas in some inverse ratio to the 

 heat developed in the wire. It was also shown, by a thermo- 

 meter placed at a given distance, that the radiated heat was in 

 a direct ratio with the visible heat. 



Although the phenomena were apparently abnormal, there 

 were many known physical agencies by which they might 

 possibly be explained, such as the different specific heats of 

 the surrounding media, their different conducting powers for 

 electricity, or the varying fluency or mobility of their par- 

 ticles which would carry off the heat by molecular currents 

 with different degrees of rapidity. 



The investigation of these questions will form the subject 

 of this paper. 



An apparatus was arranged (see fig. i). Two glass tubes, 

 A and B, of 0*3 inch internal diameter and i'5 inch length, 

 were closed with corks at each extremity ; through the corks 

 the ends of copper wires penetrated, and joining these were 

 coils of fine platinum wire, one-eightieth of an inch diameter 

 and 37 inches long when uncoiled. Tube A was filled with 

 oxygen, tube B with hydrogen, and the tubes thus prepared 



