204 Dr. Woods’s Remarks on 
duced; and therefore one of the combining bodies might evolve 
from itself the heat of combination ; whereas the theory I pub- 
lished in 1852 divests particles of any influence except that of les- 
sening the distance between themselves, or of destroying volume, 
as they come together, and so that at least two particles of matter 
are essential to its production. In his theory each body en- 
gaged in chemical action is said to give out heat by change of 
condition; in mine the volume only, or distance between the 
constituent particles, is supposed to bealtered. I still think the 
latter theory is the more reasonable, and more in accordance with 
our present scientific knowledge. 
M. Deville seems to reject this latter theory, because the 
contraction arising in chemical combination is not equivalent to 
the expansion or heat produced; and he calculates the contrac- 
tion when oxygen and hydrogen unite, to show that it is not of 
the same value or extent as the increase of volume given to other 
bodies as the accompanying or opposite movement. He also 
shows how chlorine and hydrogen unite without contraction at all; 
yet that expansion in other bodies or rise of temperature is the 
result. This apparent argument against the theory, however, dis- 
appears when it is considered that the particles whose combination 
evolves the heat are not the same as those which determine the 
volume. When oxygen and hydrogen unite, these elemental 
gases themselves, by coming together, cause other bodies to 
expand, and so are said to give rise to heat; but the volume 
attained by the compound they produce is determined by the 
distance between, not the oxygen and hydrogen, but between 
the particles of the water that results. In order, therefore, to 
calculate the contraction which causes the heat, we should know 
what takes place between the constituents of the compound: 
the bulk or volume of the compound itself tells nothing. 
An argument, therefore, for some necessary change of state 
in combining bodies as the cause of heat, drawn from the appa- 
rent want of coincidence between the contraction on the one 
hand and the heat or expansion on the other, is valueless. 
Besides, I have shown (Phil. Mag., January 1852) that the co- 
efficient of expansion increasing with the dilatation, the nearer 
particles are to each other, the greater 1s the effect they produce 
by a given contraction in causing expansion in other bodies; so 
that it is not only necessary to know the amount of contraction 
amongst the constituents of a compound at the time they com- 
bine, but also the distance they ultimately arrive at with respect 
to each other, before we can calculate the amount of heat they 
ought to produce *. 
* In the last edition of Grove’s ‘Correlation of the Physical Forces,’ 
when speaking of the theory I brought forward in 1852, to account for the 
