1888.] Professor Dewar on PhosjpJiorescence and Ozone. 557 



WEEKLY EVENING MEETING, 



. Friday, June 8, 1888. 



Sir Frederick Bramwell, U.C.L. F.E.S. Honorary Secretary and 

 Vice-President, in the Chair. 



Professor Dewar, M.A. F.E.S. M.BJ. 



Phosphorescence and Ozone. 



In spectroscopic observat'ons the experimenter is often much puzzled 

 by the phenomena presented in high vacua, and the perplexity is 

 largely due to tbe fact that we are unacquainted witli the chemical 

 changes which take place under such conditions. Special apparatus 

 has to be devised for the purpose of attempting to solve some of these 

 questions. Friction, heat, light, and electricity, will stimulate certain 

 bodies, and cause them to become phosphorescent, and cooling the 

 body may prevent the continuance of the luminosity. Again, by 

 cooling the centre of a plate which has been coated with sulphide of 

 calcium, light will make it phosphorescent everywhere but in the place 

 it bas been cooled. Heat increases the luminosity at first, but it after- 

 wards dies out more quickly than where the plate has not been 

 heated. 



Geissler was the first to discover that phosphorescence is sometimes 

 set up in residual gases in vacuum tubes. This was illustrated by 

 sending a discharge through a series of vacuum bulbs, in which the 

 traces of gas remained luminous for about five seconds after the dis- 

 charge had ceased ; when one of the bulbs was heated, on passing the 

 discharge once more, that bulb alone remained dark. Becquerel and 

 others investigated these phenomena ; some of the inquirers came to 

 the conclusion that they were produced only by oxygen compounds ; 

 others thought them to be due to some drying agent used in the 

 construction of the bulbs. 



Ozone is a very unstable body, which cannot be kept unless 

 produced at a low temperature; its boiling-point is about —100^ C, 

 and at this temperature it is a blue liquid which exhibits high 

 absorbent powers in the luminous part of the spectrum. At low 

 temperatures substances may be dissolved in it, with which it explodes 

 at high temperatures ; bisulphide of carbon is one of these substances. 

 On a former occasion I have shown that at — 150° C. phosphorus does 

 not combine with liquid oxygen, neither does sodium nor potassium, so 

 that the absence of chemical combination between ozone and oxidisable 

 substances is another proof of the negation of chemical combination 

 at low temperatures. Smell is one of the most delicate tests of the 

 presence of ozone, but inapj^licable in the instance of the contents 

 of a vacuum tube ; the investigator has then to resort to chemical 

 means and the study of the absorption spectra. In making ozone 

 from oxygen, low pressures and the presence of moisture favour the 



2 p 2 



