676 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1907. 
his talent for generalizing, he knew how to draw observations of a 
general application. It is thus that the idea of preliminary work, 
necessary for bringing about reactions, corresponds in the language 
of this time to an elevation of temperature necessary to overcome 
chemical resistance. He showed likewise that it is not the reactions 
producing the most stable system that are found, but unstable, inter- 
mediary systems. The principle of the appearance of the unstable 
forms before the stable forms is found again here, a principle which 
has been quite accurately established in these later years. Moreover, 
all the modern physico-chemists have drawn from the numerous 
thermo-chemical documents accumulated by Berthelot, and some of 
them have even at times reproduced his researches, but in a language 
corresponding to the physico-chemistry of these later years. 
Lam convinced for my part that it is particularly through thermo- 
chemistry that Berthelot acquired that truly extraordinary under- 
standing of chemical phenomena by which he seemed almost to domi- 
nate and command them. 
Thermo-chemistry was destined to lead Berthelot to the study of 
explosives. His position as president of the commission of scholars 
organized by those in command of the national defense during the 
siege of Paris had given him an opportunity to become initiated into 
the knowledge of these products. The various tasks that he accom- 
plished in this field either alone or in collaboration with members of 
the commission on powders and saltpeters, have been brought together 
in great part in his treatise “Sur la force des matiéres explosibles 
d’aprés la Thermochimie.” I should like to speak here simply of his 
“ studies of genius,” to use the expression of Nernst, on the explosive 
wave. Ina mixture of oxygen and hydrogen, for example, the combi- 
nation propagates itself in the form of a wave all the factors of which 
can be defined in advance when the properties of the exploding mix- 
ture are known. The surface of this wave, which is the seat of the 
combination, propagates itself with a speed much greater than that of 
sound, 2,800 meters in the case of oxygen and hydrogen, so that the 
influence of the cooling of the surfaces has no time to become effective. 
Besides, the speed itself is constant and independent of the nature of 
the tube which contains the mixture. The surface of the wave is at 
an extremely high temperature and exerts a strong pressure in its 
passage, a pressure which may easily be registered by placing pres- 
sure gauges in the path of the wave. 
The explosive wave has been the means of realizing the highest 
temperatures (4,000 degrees), but the products of combustion remain 
at this temperature for only a very short time. Berthelot and Vieille, 
in some extremely remarkable experiments, have used the explo- 
sive wave for furnishing quantitative evidence on the properties of 
gas at temperatures as high as 4,000 degrees. Among the numerous 
