Dr Gardner on the Action of Light on Vegetables. 77 



it was tacitly admitted, on the authority of Senebier, that the 

 chemical^ or blue ray, was most active. Professor Morren 

 (Annal. des Scien. Nat., Oct. 1832) ascribed it to the luminous 

 colours, more especially the rays which had passed through 

 bright yellow and orange glasses. Dr Daubeny (Phil. Trans. 

 1836) in his valuable researches, arrived at the same conclu- 

 sion. The next investigator, Dr Draper (Jour. Franklin 

 Inst. 1837) obtained better results in yellow than blue light. 

 Mr Hunt, in 1840 (Lond. and Edin. Phil. Mag. April), re- 

 sumed the question, and published the most decided results 

 (p. 272), to the effect, that blue light alone causes the green 

 colour of plants, and that the yellow and red rays, " destroy 

 the vital principle in the seed^ In 1841, he was one of a 

 committee appointed by the British Association, to report on 

 this subject, and in a subsequent conversation at the late meet- 

 ing of that body, has repeated his statements. Being the last 

 writer, his results have given a prominence to the doctrine, 

 that chlorophyl is produced by the blue rays, so as to mislead 

 Professor Johnston in his agricultural lectures, and Professor 

 Graham (Chemistry, p. 1013). 



(4.) In September 1840, I repeated Mr Hunt's experi- 

 ments in Virginia, and obtained dissimilar results. A known 

 number of turnip seeds were sown, and every grain germinat- 

 ed in the yellow and red rays. The greenest plants were 

 found in yellow light. Every condition was favourable, and 

 the results well characterised, but my reason for deferring the 

 publication arose from a conviction that the use of solutions 

 and coloured glasses was objectionable, and that no perfect re- 

 sults could be obtained except with the spectrum. Plants ex- 

 posed to light which has permeated cobalt glass, are not placed 

 in blue rays, but in red, yellow, green, blue, indigo, and \iolet, 

 in proportions differing with the tone of colour, and thickness 

 of the material. The effect may therefore be produced by any 

 of these rays, or by their peculiar combination. (See Sir J. 

 F. W. Herschel's paper in the Philosophical Transactions for 

 1840, p. 24, on " The combined action of rays of different de- 

 grees of refrangibility.") 



(5.) I shall not attempt to explain the discrepancy between 



