316 Sir William Siemens [April 27, 



Dr. Huggins and otliers, apj)lying this mode of research to other 

 heavenly bodies, including the distant nebulae, had extended our 

 chemical knowledge of them in a measure truly marvellous. 



Solar observation had thus led to an analytical method by which 

 chemistry had been revolutionised ; and it would be, in the lecturer's 

 opinion, through solar observation that we should attain to a much 

 more perfect conception of the nature and effect of radiant energy in 

 its three forms of heat, light, and actinism, than we could as yet 

 boast of. The imperfection of our knowledge in this respect was 

 proved by the circumstance that whereas some astronomers and 

 physicists, including Waterston, Secchi, and Ericsson, had, in following 

 Sir Isaac Newton's hypothesis, attributed to the sun a temperature of 

 several millions of degrees Centigrade, others, including Pouillet and 

 Vicaire, in following Dulong and Petit, had fixed it below 1500° 0. 

 Between these two extremes, other determinations, based upon different 

 assumptions, had fixed the solar temperature at between 60,000"^ and 

 9000^. 



The lecturer having conceived a process by which solar energy 

 may be thought to a certain extent self-sustaining, had felt much 

 interested for some years in the question of solar temperature. If 

 the temperature of the solar photosphere should exceed 3000° C, 

 combustion of hydrogen would be prevented by the law of dissocia- 

 tion, as enunciated by Bunsen and Sainte Claire Doville ; and his 

 speculative views regarding thermal maintenance must fall to the 

 ground. To test the question, he in the first place mounted a 

 parabolic reflector on a heliostat with a view of concentrating solar 

 rays within its focus, which, barring comi^aratively small losses by 

 absorption in the atmosphere and in the metallic substance of the 

 reflector, should reproduce approximately the solar temj^erature. By 

 introducing a rod of carbon through a hole at the apex of the reflector 

 until it reached the focus, its tip became vividly luminous, producing 

 a light comparable to electric light. When a gas burner was 

 arranged in such a way that the gas flame played across the 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 dissocia- 

 tion is also reached. 



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

 whether terrestrial sources of radiant energy were capable of imitating 

 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 Eoyal Society and the Koyal 

 Institution two years ago. By these researches 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 



