900 Prof. Wood and Mr. Hemsalech on Fluorescence 



being excited by radiations of some kind which came from 

 the spark. The luminosity had the form of a narrow vertical 

 jet, and its spectrum, photographed with a small quartz 

 spectrograph, showed the so-called " water-band " of the 

 oxy-hydrogen flame and the ultra-violet bands of nitrogen. 

 The intensity of the radiation was found to be much greater 

 in an atmosphere of nitrogen and much less in one of 

 oxygen. A thin plate of fluorite (1 mm. thick) placed over 

 the hole abolished the phenomenon of the luminous jet 

 completely, from which it was inferred that the excitation 

 was not due to the Schumann waves. It was thought that 

 either luminous molecules were shot out from the spark, or 

 that some sort of corpuscular radiation was responsible for 

 the excitation. The spectrum of the jet was independent of 

 the nature of the metal plate or the lower electrode, and no 

 trace of any of the spark-lines appeared in it, if the gas was 

 free from dust. It was found necessary to exercise great 

 precautions to prevent the formation of dust particles, or 

 nuclei, which scatter the light of the spark, and modify the 

 spectrum of the jet. Metallic dust is given out by the spark, 

 and ultra-violet light causes a cloud to appear in some gases, 

 so that when working in closed chambers, there must be a 

 continuous supply of fresh clean gas. 



In the winter of 1910 the writers of the present paper 

 commenced an investigation of the subject but came to no 

 very definite conclusions, though some new and interesting 

 phenomena were discovered. It was found, for example, 

 that if a narrow current of air or dry steam was blown across 

 the luminous jet, the luminosity vanished at the spot tra- 

 versed by the moving air (or steam) current, but retained its 

 full luminosity both above and below the moving gas stream. 

 It was also ascertained beyond any doubt that the luminous 

 material did not come from the spark, for if a stream of 

 C0 2 , hydrogen, or coal-gas Was directed across the jet, the 

 moving gas current emitted ultra-violet at the point at which 

 it crossed the jet, the spectrum of the emitted light differing 

 in each case. It was also proved that no deviation was 

 produced by a magnetic field. 



These results were not published at the time, though they 

 were alluded to in a reply (Phys. Zeit. xiii. p. 32, 1912) made 

 by one of us to a criticism by Steubing (Phys. Zeit, xii. 

 p. 6'26, 1911), who claimed to have shown that the jet was 

 nothing but light scattered by dust. His experiments were 

 badly carried out in an apparently hasty manner, and he 

 inferred that, since the " water-band " can be found in the 

 spectrum of some sparks, the jet must come from the spark, 



