Again, in the case of hydrogen and air exploded at constant
volume, it has been shown that whereas without " dissociation "
the temperature attained should be 4,000° C. actually it only reaches
about 2,000° C.
It is, therefore, evident that dissociation plays an important part
in combustion, and that the temperatures of compound gases are
thereby 'limited. The general opinion of scientific authorities
appears to be that the whole cause of the heat suppression is not
explained on the dissociation theory, but that dissociation is only
one of the contributing causes.
It is of interest to notice in comparing the results of tests upon

♦ Lefona sur la Dissociation professies devant la Societe Ghimique de Paris,
1864.
EXPLOSION AND COMBUSTION IN THE ENGINE 89

wide ranges of explosive mixtures, at different pressures and tem-
peratures, that the amount of apparent heat suppression is
about the same in all cases ; thus for the richest mixture given in
Table XVI this amount as indicated by the ratio of the observed
to the calculated pressures and temperatures is nearly the same
as for the weakest mixture. This and the fact first mentioned
by Schottler,* that the expansion curve does not suddenly vary
its course and shape as it should do in accordance with the above
theory, are evidence that dissociation by itself does not fully account
for the actual changes observed.
(2) Influence of the Cylinder Walls.
The whole subject of " wall action," as it is termed, is of great
importance to students and designers of internal combustion
engines, apart from affording a reasonable explanation of the
observed facts of heat suppression already alluded to.
