428 Dr. J. W. Draper on the Distribution of 



hydrochloric acid by a small voltaic battery. This instrument, 

 modified to suit their purposes, was used by Professors Bunsen 

 and Roscoe in their photometrical researches. Many of my ex- 

 periments were repeated by 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 energy. The maximum is 

 in the indigo, the action there being more than 700 times as 

 powerful as in the extreme red. 



6th. Case of the bending 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, however, was the first 

 to examine the details of this phenomenon in the spectrum ; 

 his memoir is in the Philosophical Magazine (Jan. 1844). 

 When seeds are made to germinate aud grow for a few days in 

 darkness, they develope vertical stems 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 spec- 

 trum turn towards the indigo ; those in the indigo bend to the 

 approaching ray. Removed into darkness, they recover their 

 upright position. These movements are the most striking of 

 all actinic phenomena; I have often witnessed them with ad- 

 miration. 



Dr. Gardner's experiments were repeated and confirmed by 

 M. Dutrochet, who, in a report to the French Academy of 

 Sciences (Comptes 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 coloured rays of the 

 spectrum. 



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

 respecting the bleaching or decolorization of cblorophyl by 

 light. He used an ethereal solution of that substance. 



"The first action of light is perceived in the mean red rays; 

 and it attains a maximum incomparably greater at that point 

 than elsewhere. The next part affected is in the indigo ; and 

 accompanying it there is an action from + lO'S to + 36*0 of 

 the same scale (HerscheFs), begmning abruptly in Fraunhofer's 

 blue. So striking is this whole result, that some of my earlier 

 spectra contained a perfectly neutral space from —5*0 to +10*5, 

 in which the chlorophyl was in no way changed, whilst the 

 solar picture in the red was sharp and of a dazzling white. The 

 maximum in the indigo was also bleached, producing a linear 



spectrum as follows,—— .___ .^ in which the orange, 



yellow, and green rays are neutral. These, it wall be remem- 



