Dr. Gardner on the Action of Light upon Vegetables. 11 



(29.) The investigation of these important problems has cost 

 me much labor, but the following results will show, that a satis- 

 factory solution has been attained. 



A trough of plate-glass, containing persulphocyanide of iron, 

 which has the property of absorbing the tithonic rays of the in- 

 digo space, and allowing indigo light to pass, was placed before 

 a small aperture, made in the side of a suitable box. The proper 

 place for the hole, was determined by receiving the analyzed 

 spectrum on a Daguerre plate, resting against the box. In a few 

 minutes, two stains were observed, with an interval between 

 them, corresponding to the place of the indigo light. The inac- 

 tive space was marked on the wood, and a perforation made in 

 its centre, without deranging the adjustment, so that the aperture 

 continued to admit detithonized light. Plants placed in this box 

 were bent in two hours, whilst a crop illuminated by indigo rays, 

 which had not been transmitted through the solution, did not 

 move with much more activity, although one crop was exposed 

 to the maximum of the indigo tithonic rays, and the other placed 

 in detithonized light. 



(30.) Solution of bichromate of potash, intercepts nearly all 

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

 A crop of turnip seedlings was introduced into a box and illumi- 

 nated by the yellow rays of the spectrum, analyzed by this solu- 

 tion. A Daguerre plate was also introduced, to serve as a test of 

 chemical action. In two hours and a half the plants 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 differ- 

 ent. If the bending had been due to tithonicity, 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-hundreth part, as measured by Dr. Draper's tithonome- 

 ter.* 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. 



* Tithonometer — an instrument for measuring the chemical force of rays, by the 

 union of chlorine and hydrogen. 



