SIR WILLIAM SIEMENS, F.R.S. 447 



focal area, combustion appeared to be retarded, but was not 

 arrested, showing that the utmost temperature attained in the 

 focus did not exceed materially that producible in a Deville oxy- 

 hydrogen furnace, or in the lecturer's regenerative gas furnace, in 

 which the limit of dissociation is also reached. 



Having thus far satisfied himself, his next step was to ascertain 

 whether terrestrial sources of radiant energy were capable of imi- 

 tating solar action in effecting the decomposition of carbonic acid 

 and aqueous vapour in the leaf -cells of plants, which led him to 

 undertake a series of researches on electro-horticulture, extending 

 over three years, a subject he had brought before the Royal 

 Society and the Royal Institution two years ago. By these re- 

 searches he had proved that the electric arc possessed not only 

 all the rays necessary to plant-life, but that a portion of its rays 

 (the ultra-violet) exceeded in intensity the effective limit, and had 

 to be absorbed by filtration through clear glass, which, as Professor 

 Stokes had shown, produced this effect without interference with 

 the yellow and other luminous and intense heat rays. He next 

 endeavoured to estimate the solar temperature by instituting a 

 comparison between the spectra due to different known luminous 

 intensities. Starting with the researches of Professor Tyndall on 

 radiant energy, supplementing them by experiments of his own 

 on electric arcs of great power, and calling to his aid Professor 

 Langley, of the Alleghany Observatory, to produce for him a 

 complete spectrum of an Argand burner, he concluded that with 

 the temperature of a radiant source, the proportion of luminous 

 rays increased in a certain ratio ; whereas in an Argand gas- 

 burner only 2i per cent, of the rays emitted were luminous and 

 mostly red and yellow, the most brilliant portion of a gas flame 

 emitted 4 per cent., as shown by Tyndall, the carbon thread of an 

 incandescent electric light between 5 and 6 per cent., a small 

 electric arc 10 per cent., and in a powerful 5000-candle electric 

 arc as much as 25 per cent, of the total radiation was of the 

 luminous kind. Professor Langley, in taking his photometer and 

 bolometer up the TVhitley mountains, 18,000 feet high, had proved 

 that of the solar energy not more than 25 per cent, was luminous, 

 and that the loss of solar energy sustained between our atmo- 

 sphere and the sun was chiefly of the ultra-violet kind. These 

 rays, if they penetrated our atmosphere, would render vegetation 



