146 PROFESSOR H. B. DIXO^l OX THE RATE OF EXPLOSION IX GASES. 
Table XXXVII.—Rate of Explosion of Hydrogen and Chlorine. 2nd Series. 
1718 
1727 
1725 
Mean 1723 
If the preliminary series of experiments with the dry gases is regarded as of equal 
value with one of these pairs, the mean rate of explosion is found to be 1729 metres 
per second. 
The heat of combinfition of hydrogen and chlorine was found by Fayre and 
SiLBERMANN to be 23,780 calories ; by J. Thomsen it was found to be 22,000 calories. 
Berthelot gives one determination 22,100 calories. The mean of these numbers 
is 22,630 calories. Taking Thomsen’s number for the heat of combination, the mean 
rate of translation of the products of combustion according to Berthelot’s formula 
(corrected to 13° C.) is found to be 1551 metres per second; taking the mean 
number 22,630 calories for the heat of combination, Berthelot’s formula gives 
1571 metres for the mean rate. The rate of explosion of hydrogen and chlorine 
under ordinary conditions is, therefore, considerably faster than the rate given by 
Berthelot’s formula. The velocity of the sound-wave (2) calculated by my formula 
is 1805 or 1830, according as the lower or higher heat of combination is taken. 
The rate of the explosion thus falls appreciably below the calculated velocity of the 
sound-wave. Since Berthelot contends that his formula gives the maximum 
velocity of explosion, his hypothesis fails in the case of the explosion of hydrogen 
and chlorine. On the other hand, two questions arise: (1) Hoes dissociation.occur 
in the explosion ? (2) On diluting the mixture will the rate of explosion approach 
the velocity of the sound-wave ? 
When the hydrochloric acid gas, produced by the explosion in the long tube, was 
driven out by a sti'eam of hydrogen through a washing apparatus containing potassium 
iodide, a quantity of iodide was liberated, showing the presence of free chlorine in the 
products of combustion. And when the gases were driven out by a stream of carbonic 
acid through a washing apparatus containing caustic soda, free hjMrogen was found 
to be present also. Even after passing the cuiTent for many hours the gases coming 
from the electrolytic cell, and issuing from the further end of the explosion tube, 
were always founci to contain a slight excess (amounting to between ’2 and ‘5 per 
cent.) of hydrogen. The presence of free chlorine in the vessel after the explosion 
was thus a proof of dissociation occurring in the explosion, and the determination of 
this free chlorine by the liberation of iodine afforded a ready means of measuring the 
volume of gases left unburnt. About 1 per cent, of the combustible gases were found 
uncombined. 
