10 Dr. Gardner on the Action of Yellow 



30. Solution of bichromate of potash intercepts nearly all 

 tithonic matter, but permits the free passage of luminous rays. 

 A crop of turnep-seedlings was introduced into a box and illu- 

 minated by the yellow rays of the spectrum analysed by this 

 solution. A Daguerre plate was also introduced, to serve as 

 a test of chemical action. In 2^ hours the plants were all 

 equally bent, and the plate but slightly stained at one side. A 

 group of similar plants exposed in the same place, without the 

 solution, were inclined in a period of time not materially dif- 

 ferent. If the bending had been due to tithonicity, the seed- 

 lings should have moved towards the place where the plate was 

 stained. 



31. The tithonic activity of rays transmitted through the 

 above solution, from an Argand lamp, is diminished to less 

 than 2^o l h P art > as measured by Dr. Draper's instrument. 

 But plants were bent in light from this source which had tra- 

 versed the solution in a period not much greater than that re- 

 quired in the full blaze of the lamp. This result alone is 

 abundantly sufficient to decide the question, and show the 

 total inactivity of the tithonic rays in producing these vege- 

 table movements. 



32. That the bending is not due to heat appears from the 

 following considerations : — the action is greatest in those parts 

 of the spectrum which give evidence of least heat. The axis 

 is approached on one side by the red, orange, yellow and 

 green, and by the violet and lavender plants on the other, 

 which is a phaenomenon that cannot be explained on the sup- 

 position that heat is the active agent. Plants shut from the 

 light of an Argand lamp by a plate of copper foil do not in- 

 cline to the warm metal. 



Finally, the moonbeams, even without condensation, are 

 capable of producing extensive bending in one or two hours. 

 This result is conclusive of the question, for no trace of ca- 

 loric can be found in the moon's light. 



33. As far, therefore, as the presence of heat can be deter- 

 mined by thermoscopes, or the tithonic rays by argentine 

 compounds, and the union of chlorine and hydrogen, we are 

 justified in concluding that the movements of plants are ef- 

 fected by a totally different agent. Light only remains in the 

 spectrum, so far as we know, and to it therefore I refer the 

 movements under consideration. 



34<. This conclusion is of deep interest, inasmuch as it is 

 thejirst case of a movement, perceptible to the eye, being traced 

 to the unaided action of light. That this imponderable pro- 

 duced molecular changes was readily admitted, but its influ- 

 ence in bringing about palpable movements of considerable 



