Dr Gardner on the Action of Light on Vegetables, 87 



were all equally bent, and the plate but slightly stained on one 

 edge. A group of similar plants, exposed in the same place, 

 without the solution, were inclined in a period of time not 

 materially different. If the bending had been due to tithoni- 

 city, the seedlings should have moved towards the place where 

 the silver was stained. 



(31.) The tithonic activity of rays transmitted through the 

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

 one two-hundredth part, as measured by Dr Draper's tithono- 

 meter.* But plants were bent in light from this source which 

 had traversed the solution, in a period of time not much 

 greater than that required in the full blaze of the lamp. 



This result alone is abundantly sufficient to decide the ques- 

 tion, and shew the total inactivity of the tithonic rays in pro- 

 ducing these vegetable 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 plants from the red, orange, 

 yellow, and green, and by those from the violet and lavender 

 on the other, which is a phenomenon altogether inexplicable, 

 on the supposition that heat is the active agent. 



Plants shut from the light of an argand lamp, by a plate o£ 

 copper foil, do not incline 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 caloric 

 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 eifect- 

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

 spectrum^ so far as we knorv ; and to it, therefore, I refer the 

 motions under consideration. 



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



* Tithonometer — an instrument for measuring the chemical force o 

 rftys, by the union of chlorine and hydrogen. 



