OF THE FLAME IX THE EXPLOSION OF GASES. 
347 
where the luminosity of the lines was in marked contrast, and disappeared entirely 
when the films were less sensitive, or the contrast of luminosity was diminished for 
other reasons, as in figs. 40 and 41 and many other photographs (which are not 
reproduced), I came to the conclusion that the effect was due to halation on the 
photograph, the brighter lines being enlarged. 
But in 1900 Le Ciiatelier, relying on the same kind of evidence, put forward the 
view that the wave of detonation starts in front of the variable wave (which is 
increasing in velocity), and originates in an invisible wave, which is proceeding in 
front of the visible wave and with a velocity ecpial to it. He says ;— 
“ II n’y a, dans aucun cas, continuite entre la periode variable et I’onde explosive. 
Celle-ci jorend naissance a une certaine distance en avant de la flamme a vitesse 
variable, a O'Oo metre dans le melange C^Hj+Oo. Ce fait est accuse par un ressaut 
de la courbe photographique enregistree; . . . . Dans cette periode variable, la 
flamme est precedee d’une onde compiimee qui marche devant elle avec une vitesse 
egale, comme le font a la surface de I’ecran les ondulations qui precedent la prone 
d’un navire. Une fois fonde exjfiosive developpee, les deux phenomenes se super- 
posent, c’est-a-dire que le front de I’onde comprimee coincide avec la tranche gazeuse 
en combustion, an lieu de la preceder.” 
This defiiiite judgment of the brilliant French experimenter compelled us to 
re-examine the question. We attempted at first to decide the matter by photo¬ 
graphing an explosion as it passed from a less luminous mixture into a more luminous 
one ; but we could not succeed in making the transition sufficiently sudden. However 
quick we were in pushing a short column of C 3 N 3 + O 3 into one end of a tube filled 
with 2 H 3 -j- Oo, and firing the latter, the gases had diffused enough to prevent any 
abrupt change in the brightness of the explosion. We did, however, succeed in 
obtaining sudden changes of brightness by introducing a layer of “ Welsbach” salts 
(a mixture of thoria and ceria), and liaving the rest of the tube quite clean. Although 
to raise the salt from the glass and to render it incandescent must take some time, 
nevertheless the photograph shows a small iDut distinct break in tlie line of detonation 
similar to that in question. Figs. 70 and 71 sliow the j^assage of tlie wave of 
detonation in electrolytic gas througli such a “salted” tube. If the salts could ])e 
raised instantaneously, it is possible that the break would be as well-marked as any 
observed in the development of the detonation. 
It is of course easy to show the enlargement due to brightness. If a tube is filled 
with a mixture giving a luminous explosion, and the explosion is photographed while 
half the tube is covered over, and if the tube is then filled with a mixture giving a 
less luminous explosion, which is photographed on the same film while the first half of 
the tube is covered, a photograph is obtained (fig. 72) which shows a greater dis¬ 
continuity than any of those in question. 
Another way of showing the same thing is to photograph a thin platinum wire 
2 y 2 
