DECOMPOSITION OF WATER BY HEAT. 329 



this is strikingly shown with zinc. If the arc be taken between 

 zinc points in an exhausted receiver, a fine dark powder, 

 nearly black, is deposited on the interior, which, when col- 

 lected, proves to be pure zinc, and on the application of a 

 gentle heat takes fire in the open air and burns into the white 

 oxide : to casual observation the zinc would appear to be 

 burned twice. The experiment appears to me to present an 

 argument in favour of the dynamic theory of heat. 



With charcoal, on the other hand, there is little or no 

 deposit, but the charcoal continually yields carbonic oxide 

 and hydrogen, and this for hours after the presence of water 

 would be deemed impossible. I have taken the arc between 

 pieces of well-burned charcoal for eight or nine successive 

 hours, and there was still gas generated ; indeed, it appeared 

 to be given off as long as there was any charcoal remaining, 

 and a conversion of the carbon into inflammable gas might 

 have been supposed. Much still remains to be done with this 

 powerful agent, the voltaic arc : where, however, the object is 

 simply to expose gases to an intense heat, the ignition of a 

 conjunctive wire of platinum is more simple in its application, 

 more uniform in its action, and instead of requiring a powerful 

 battery, the effect can be satisfactorily produced by five or six 

 cells, in many cases by two. 



The heat is not so intense as that of the arc, but as it can 

 be brought to within a few degrees of the fusing-point of pla- 

 tinum, it is far more intense than any heat usually employed 

 in laboratories, certainly than any which can be applied to 

 minute, I may say microscopic portions of gas or vapour. 



In conclusion, I must express my sincere thanks to the 

 Managers of the London Institution, for having permitted me, 

 as an honorary member, to carry on these experiments in the 

 laboratory of the Institution. 



