16 
MR. GROVE ON THE DECOMPOSITION OF WATER BY HEAT. 
tive discharge, which in many points may be regarded simply as very intense ignition; 
but I am only aware of two experiments of his on voltaic ignition ; one, in which he 
employed it in an exhausted receiver to examine to what extent the radiation of heat 
was carried on in vacuo ; and another, already alluded to, in which, by immersing a 
portion of an ignited wire in water, he observed that it conducted in some inverse 
ratio to its heat. 
I have made a vast number of experiments on the voltaic arc or disruptive dis- 
charge, in various media* ; when this is taken in a medium incapable of acting 
chemically on the electrodes, the phenomena are those of intense ignition of the 
terminals, which are dissipated in vapour and condensed upon the interior of the 
vessel in which the discharge is taken. I have examined some of these deposits, and 
they appear to consist of the metal of the terminals in a finely-divided state ; 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 
collected, 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 platinum, 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 expe- 
riments in the laboratory of the Institution. 
* Philosophical Magazine, June 1840. Literary Gazette and Athenseum, February 7, 1845. 
London Institution, 
August 21 , 1846 . 
