MEMOIR XXIX.] CHEMICAL FORCE IN THE SPEQTR^M. 



hydrochloric acid by a small voltaic battery;/ Thi^ip- 

 strument, modified to suit their purposes, was tf^^4 ky?> 

 Professors Bunsen and Roscoe in their photometrica^re- 

 searches. Many of my experiments were repeated 05^ 

 them (Transactions of the Royal Society, 1856, 1857). 



In Table III. of my Memoir above referred to, it is 

 shown that this mixture is affected by every ray of the 

 spectrum ; but by different ones with very different en- 

 ergy. The maximum is in the indigo, the action there 

 being more than 700 times as powerful as in the ex- 

 treme red. 



th. Case of the Sending of the Stems of Plants in the 



Spectrum,. 



It is a matter of common observation that plants tend 

 to grow towards the light. Dr. Gardner was, however, 

 the first to examine the details of this phenomenon in 

 the spectrum. His Memoir is in the Philosophical Mag- 

 azine (Jan., 1844). When seeds are made to germinate 

 and grow for a few days in darkness, they develop verti- 

 cal stems, very slender and some inches in length. These, 

 on being placed so as to receive the spectrum, soon ex- 

 hibit a bending motion. The stems in other parts of 

 the spectrum turn towards the indigo; those in the in- 

 digo bend to the approaching ray. Removed into dark- 

 ness, they recover their upright position. These move- 

 ments are the most striking of all actinic phenomena. I 

 have often witnessed them with admiration. 



Dr. Gardner's experiments were repeated and con- 

 firmed by M. Dutrochet, who, in a report to the French 

 Academy of Sciences (Oomptes - Rendus, No. 26, June, 

 1844), added a number of facts respecting the bending 

 of roots from the light, which he found to be occasioned 

 by all the colored rays of the spectrum. 



In Dr. Gardner's paper there are also some interesting 



