at the Royal Institution, 1908-1916 745 



when hydrogen was introduced into the arc, although the iron was 

 still present. Mr. De la Rue's photographs of the effects of the 

 electric discharge upon the minute residue in rarefied tubes were 

 exhibited and imitations of the Aurora Borealis were produced in 

 tubes not so much exhausted. The polarization was as strikingly 

 demonstrated in the electric arc as it is in a Grove's cell. The effects 

 of the discharge at high and low pressure were next illustrated. 

 Having exhibited the brilliant hydrocarbon gases, acetylene and 

 olefiant gas, the chief illuminating constituents in ordinary coal gas, 

 the Professor produced acetylene in the electric arc by the introduc- 

 tion of hydrogen ; and in a similar manner he produced hydrocyanic 

 acid gas by introducing nitrogen and hydrogen into the arc ; he also< 

 exhibited the beautiful peach-coloured flame of cyanogen rendered 

 more brilliant by the introduction of oxygen. Remarks were made 

 on the important effects of the silent electric discharge in nature, in 

 causing condensation instead of decomposition." 



A noteworthy series of observations, foreshadowing the method 

 now used in producing nitric acid from the atmosphere, was de- 

 scribed in the lecture on January 16, 1880, showing the effects pro- 

 duced by subjecting air to the action of an electric arc within a 

 cooled metallic vessel through which air Was continuously circulated. 

 The arc produced by a discharge of a high intensity, from a De 

 Meriten's machine, between the poles of a Jablockoff candle from 

 which the insulator had been removed, gave the best yield of nitrous 

 compounds, a long Siemens arc being superior to the short. It was 

 also shown to be advantageous to remove the products from the 

 heated region by a rapid current of air, inferior results being ob- 

 tained if the current were slow. 



From radiation experiments referred to in the lecture, the tem- 

 perature of the positive pole of the Siemens nrc was estimated at 

 about 6000° 0. 



A lecture which does not fit into the general scheme may be 

 referred to here on account of the importance of the subject — " The 

 Scientific Work of Joule " — and of the demonstration Prof. Dewar 

 gave of the way in which Joule's determination of the value of the 

 mechanical equivalent of heat was led up to by the early work of 

 Count Rumford, the Founder of the Royal Institution, as well as later 

 on by that of two of its Professors — Davy and Young — both, as the 

 lecturer showed, " so near and yet so far from Joule." 



An interesting feature of the lecture, which was delivered on 

 January 24, 1895, is a memorable letter from Lord Playfair giving 

 his recollections of Joule. This contains the following noteworthy 

 ge : — 

 When Joule first sent an account of his experiments to the Royal 



