Mr. J. Dewar on the Chemical Efficiency of Sunlight. 309 



sues, it would be interesting to acquire some idea of tlie storage 

 of power effected by a given area of leaf-surface during the 

 course of a day, and to compare this with the total available 

 energy. Here we are dealing with strictly measurable quanti- 

 ties, provided we could determine the equation of chemical 

 transformation. 



Boussingault's recent observations on the amount of carbonic 

 acid decomposed by a given area of green leaf seem to me to 

 afford interesting data for a new determination of the efficiency of 

 suiilight. By experiments made between the month of January 

 and October under the most favourable circumstances in atmo- 

 spheres rich in CO^, one square decimetre of leaf was found to 

 decompose in one hour, as a mean, 5*28 cub. centims. of CO^ 

 and in darkness to evolve during the same period of time 0*33 

 cub. centim. of CO^. In other words, one square metre of 

 green surface will decompose in twelve hours of the day 63'36 

 cub. centims. of CO^, and produce in twelve hours of the night 

 3-96 cub. centims. of CO^*. 



This quantity of carbonic acid decomposed does not represent 

 the whole work of sunlight for the time, as water is simultane- 

 ously attacked in order to supply the hydrogen of the carbo- 

 hydrates. Boussingault, in summing up the general results of 

 his laborious researches on vegetable physiology, says, " Si Ton 

 envisage la vie vegetale dans son ensemble, on est convaincu que 

 la feuiile est la premiere etape des glucoses que, plus ou moins 

 modifies, on trouve repartis dans les diverses parties de Porga- 

 nisme ; que c'est la feuiile qui les elabore aux depens de Pacide 

 carbonique et de Feau.^^ — Ann. de Chemie, torn. xiii. p. 415. 

 The fundamental chemical reaction taking place in the leaf may 

 therefore be represented as follows : — 



(1) C0,0 + H^O = CO,W + 0,0 



(2) 6(CO.H2) = C^Ri^^O^ 



In the first equation carbonic acid and water are simul- 

 taneously attacked, with the liberation of a volume of oxygen 



* The rate at which the leaf functions is dependent on the luminous 

 intensity. The relative amounts, therefore, of carbonic acid decomposed 

 through the action of the different coloured rays are proportional to their 

 luminous power; and the curve of assimilation is found to follow the curve 

 of Fraunhofer. This proves that the judgment we form of equal luminous 

 impressions is in reality due to equal mechanical effects associated with the 

 different coloured rays. Professor Draper, of New York, in his recent 

 paper " On the Distribution of Heat in the Spectrum," hy dividing the 

 spectrum into two portions of equal luminous intensity, obtained identical 

 thermal effects by absorption. This does not prove that each ray has the 

 same total energy, but only that in all probability those at equal distances 

 on either side of the mean wave-length in the normal light-spectrum of the 

 sun are identical. 



