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Prof. H. B. Dixon. On the Movements 



[June 5, 



Part VII. — Further Experiments on the Initial Phases. 

 [In conjunction with B. Dawson, B.Sc, and L. Bradshaw, B.Sc] 

 1. Le Chatelier's Hypothesis of Discontinuity in the Explosion. 



Many photographs show a peculiarity at the point where a less 

 luminous line is succeeded abruptly by a more luminous one. The 

 lines photographed do not appear continuous, but the more luminous 

 line appears to start from a point not yet reached by the less luminous 

 one. The point of collision also of two waves appears to project in 

 front of the waves which are meeting. It appeared to me at first as 

 if these appearances might be due to invisible waves advancing in front 

 of the visible ones, but as I found that they only showed where the 

 luminosity of the lines was in marked contrast, and disappeared 

 entirely when the films were not sensitive, or the contrast of lumi- 

 nosity was diminished, I came to the conclusion that the effect was due 

 to halation on the photograph, the brighter lines being enlarged. 



But in 1890 Le Chatelier, 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 equal to it. 



This definite judgment of the French experimenter compelled us to 

 re-examine the question. We attempted at first to decide the matter 

 by photographing an explosion as it passed from a less luminous mix- 

 ture into a more luminous one ; but we could not succeed in making 

 the transition sufficiently sudden. 



We did, however, succeed in obtaining sudden changes of brightness 

 by introducing a laj^er of " Welshach " salts (a mixture of thoria and 

 ceria), and having 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 but distinct break in 

 the line of detonation similar to that in question. 



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 which shows a greater discontinuity 

 than any of those in question. 



Another way of showing the same thing is to photograph a thin 

 platinum wire stretched by weights, and rendered luminous by an 

 electric current. If a second wire is brought to touch the first so as to 

 divide the current, the portion of the wire which carries the whole 

 current is more luminous than the other portion, and the photographs 

 make it appear of far greater diameter. 



