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



equal to that of the original carbonic acid^ together with the forma- 

 tion of a substance having the composition of methylic aldehyde. 

 The second equation represents the condensation of this aldehyde 

 into grape-sugar. The transformation induced in (1) necessi- 

 tates the absorption of a large amount of energy ; and if we 

 neglect the heat evolved in the combination of nascent CO and 

 H^, which can be shown to be very little^ the calculated result 

 is made a maximum ; whereas the condensation of (2) being 

 attended with an evolution of heat, diminishes considerably the 

 amount of power required. Happily Frankland^s direct deter- 

 mination of the thermal value of grape-sugar leaves no doubt as 

 to the true equivalent of work done in its formation. Taking 

 the following thermal values C0,0 = 68,000, H^O = 68,000, 

 Q6i{UQ6 ^ 642,000, 1 cub. centim. of CO^ decomposed as in 

 (1) would require 6*06 gramme-units of heat, or its light- 

 equivalent, whereas the complete change into grape-sugar of 

 the same amount of carbonic acid requires only 4*78 gramme- 

 units. But, we have seen before, 1 square decimetre of green leaf 

 functions at the rate of 5*28 cub. centims. of carbonic acid assimi- 

 lated per hour; therefore 5*28 x 4-78 = 25*23 represents the 

 number of gramme- heat units conserved through the absorption 

 of light in the above period of time. Pouillet estimates the 

 mean total solar radiation per square decimetre exposed nor-« 

 mally to the sun^s rays in or near Paris per hour as 6000 

 gramme-units, so that 6000 -h 25*23 = 23-^ represents the 

 fraction of the entire energy conserved. The estimate is by no 

 means too great, as Boussingault has shown the leaf may func- 

 tion at twice the above rate for a limited time ; and as both 

 sides of the leaf are included in the measurement of the green 

 surface in his memoir, we ought to double the fraction for a 

 leaf exposed perpendicularly to the sun^s rays, increasing the 

 above number to the 120th part. 



In connexion w^ith equation (1), above given, as representing 

 the action of sunlight on the leaf, it is worthy of remark that, 

 supposing the carbonic acid and water equally efficient as ab- 

 sorbing agents of the vibratory energy (although each has a 

 specific absorption for certain qualities of rays), the decompo- 

 sition of the two compound molecules may take place conti- 

 nuously side by side, owing to the equality of the thermal equi- 

 valents of carbonic oxide and hydrogen. We already know, from 

 the laborious researches of Tyndall, how thoroughly aqueous 

 vapour retains thermal radiations; and Janssen has further 

 shown the same substance has a strong absorptive action on the 

 rays of light of low refrangibility (just those rays that are in part 

 selected by chlorophyl), producing the well-known atmospheric 

 lines of the solar spectrum. The presence, therefore, of varying 



