187 
decomposition in bodies, while positive electricity, 
like the calorific rays, occasions both decomposition 
and combination. 
456. Farther, the phenomena, which accompany 
these chemical changes, attest the great similarity in 
the operations of these subtile agents ; for light and 
heat, the characteristics of combustion, excited by the 
calorific rays, are the well-known attendants of ordi- 
nary electricity. In the low state, indeed, in which 
Galvanic electricity sometimes effects decomposition, 
no heat is perceptible ; but when it is employed in a 
state of greater intensity, Mr Davy found it rapidly 
to evaporate water, and to inflame and volatilize ni- 
trate of ammonia *. Whenever bodies, brought by 
artificial means into a high state of opposite electrici- 
ties, are made to restore the equilibrium, heat and 
light, says he, are the common consequences ; and 
they are, also, the constant result of intense chemical 
action. But where large quantities of electricity, of 
low intensity, act, or the combinations are slowly ef- 
fected, there is an increase of temperature without lu- 
minous appearance ! 
457. To the exhibition, however, of these pheno- 
mena in combustion, the presence of oxygen is ne- 
cessary ; for the calorific rays do not excite com- 
bustion in vacuo, nor in any gas deprived of oxy^ 
gen, even when the most inflammable substance is 
employed. The very same condition is required for 
the excitation of electricity. Colonel Haldane ob- 
served, that, when the Voltaic pile was placed in 
ranto, its action immediately ceased ; that in nitro- 
Phil. Trans, 1307, f Ibid. 
