Mr. J. Devvar on the Chemical Efficiency of Sunlight. 307 



character of the received definition of intensity in the theories 

 of light and sound, my correspondent preserves silence as to the 

 mode in which the defect is to be remedied. I conclude, there- 

 fore, that I have been the first to point out the true definition of 

 intensity in these theories. 



That a principle so simple, so important, lying at the very 

 threshold of the subject, should have hitherto been suffered to pass 

 without recognition, is only one of abounding proofs of the 

 crude and imperfect manner in which the theory of undulations 

 has been treated, and of the necessity which exists for its entire 

 revision, as well with the view of eradicating from it the errors 

 with which it has been allowed to be mixed up, as of placing it 

 upon a firm and intelligible scientific basis. 



6 New Square, Lincoln's Inn, 

 June 12, 1872. 



XXXVI. On the Chemical Efficiency of Sunlight. 

 By James Dewar, Esq,^ 



OF all the processes proposed to measure varying luminous 

 intensity by means of chemical effects, not one has yet 

 been expressed in strictly dynamical measure. This is owing 

 to the very small amount of energy to be measured necessitating 

 very peculiar processes for its recognition. The chemical ac- 

 tions generally induced by light are of the ^' Trigger^'' or 

 "^ Relay ''^ description — that is, bear no necessary relation to the 

 power evolved by the transformation. There is one natural 

 action of light, however, of a very diff'erent kind, continuously at 

 work in the decomposition of carbonic acid by plants, necessita- 

 ting a large absorption of energy, and thus enabling us to 

 ascertain the proportion of the radiant power retained, through 

 the chemical syntheses effected. 



So far as I am aware, the following passage, extracted from 

 Helmholtz's Lectures " On the Conservation of Energy,'^ de- 

 livered at the Royal Institution in 1864, and published in the 

 ^ Medical Times and Gazette,^ contains the first estimate of the 

 chemical efficiency of sunlight. " Now, we have seen already, 

 that by the life of plants great stores of energy are collected in 

 the form of combustible matter, and that they are collected 

 under the influence of solar light. I have shown you in the 

 last lecture that some parts of solar light — the so-called chemi- 

 cal rays, the blue and the violet which produce chemical action 

 — are completely absorbed and taken away by the green leaves 

 of plants ; and we must suppose that these chemical rays afford 



* Communicated bv tlie Author, having been read before the Royal 

 Society of Edinburgh, May 6, 1872. 



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