SI REV. G. HEXftLuW ON" THE 



et a la lumiere du gaz m'oiit conduit a ce re^^ultat : qtce les rayons 

 calonjiqnes ohsciirs ar/isseiit tres-fortement sitr la transpiration^ et 

 qiie cette influence, reJativement a celle des metres rayons da 

 spectre, est plus grande quand on se sert de la lumiere du gaz que 

 quand on opere a la lumi: re solaire^'^. 



^fter detailing his methods of experiment he concludes: — 

 " II y a entre la valour calculee de la transpiration a I'obscurite 

 et la valeur ohservee unc difference de 11 pour 100. 



^^En passant sur ces erreurs d'expericnces, on trouve d'une 

 manicre approximative que pour 100 d'eau transpiree, 70 revien- 

 nent aux rayons lumineux et ultra-violets, et 27 aux rayous 

 calorifiques obscurs." 



It is worth while to rccnll the fact that Dauheny in 1S36 2)er- 

 ceivod the importance of heat-rays. He attributed the excess of 

 transpiration under blue, j)urp]e, and the cupreous solution, in 

 great part to the lieat radiated from the screen, for ^' a hot.tle 

 filled with water, hlachened with ink to such a degree as to 

 transmit just as much light so far as could be measured by the 

 eye, as that filled Avith tl^e copper solution was found to do, 

 caused an equally considerable amount of water to be evolved by 

 the jdant . . . Now as water, witli the addition of a little ink, is 

 known to absorb the rays proceeding from all parts of the spec- 

 trum in an equal ratio, it follows that the effect produced in 

 either instance must be ascribed to the heat radiated, and not to 

 any peculiar virtue of the violet extremity in stimulating the 

 vesretativp functions." 



'* Yet," he concludes, " the presence of sume light seems essen- 

 tial to the due continuance of the process "f. 



" La flamme du gaz donne bien plus de rayons calorifiques que de rayon* 

 lumineux (Tyndall)." 



t Ann. Sci. Nat. s^r. i\ iv. (1876) p. 160. I am not aware of any other attempts 

 to ascertain the exact amount of difference between evaporation, due to heat alone, 

 and to transpiration, due to light. From some experiments (not yet sufficient for 

 publication) I have found very marked differences in the amount of water lost 

 in the same times and conditions, as well as in the rapidif?/ with which desic- 

 cation takes place under the two processes. Such, at least, was found to obtain 

 when one half of a living leaf transpired under sunlight ; while the other half, 

 having been mddtuli/ killed by scalding, could only evaporate. I hope to be 

 able to differentiate these two processes more accurately on a future occasion. 

 An important paper indirectly bearing upon this subject is one by M. Maquenne, 

 entitled, ** Eecherches sur la dt'termination des pouvoirs absorbants et diffusifs 

 des feuilles" {Annales Agrovomiqves, tom. 6, 1880, p. 321). 



A discovery of Maqtienne's (which would seem to furnish an important aid 



