On the Wave-surface in the Theory of Double Refraction. 417 



the one for the other. The second objection has apparently 

 more force, but I am able to prove that the vessels are not 

 coated with silica, but are actually composed of silica, by 

 showing that if the silica be removed, no trace of vessel re- 

 mains. I could wish to give particular importance to the 

 mode of working out this result, and to some of the facts con- 

 nected with it, because much light appears to be thrown upon 

 the nature of a new and beautiful portion of vegetable struc- 

 ture, I mean the system of, apparently, small cups, which are 

 placed along the siliceous columns of gramineous plants*. 



In order to effect the removal of silica without disturbing 

 other parts of the plant, I placed a small portion of one of 

 the lower leaves of a stalk of wheat for upwards of twelve 

 hours in caustic potash. After removing the potash by di- 

 lute muriatic acid, I mounted half of the specimen in balsam, 

 and then expelling the carbon from the other half by the aid 

 of a spirit-lamp, I inclosed this portion also in the same sub- 

 stance. These I compared with each other and with the ad- 

 joining part of the leaf in its natural state. The caustic potash 

 had effected the entire removal of the system of siliceous ves- 

 sels between the ribs of the leaf, but the small cups which 

 are duly arranged along the siliceous columns remained un- 

 disturbed. These cups, therefore, are not composed of silica ; 

 neither are they carbonaceous, for after resisting the action of 

 potash, they resist also the action of fire. This is not the 

 case with the ducts, &c. which form the ribs of the leaf; they 

 are readily carbonized and dissipated. But after the car- 

 bonaceous parts have been thus expelled, and these cups alone 

 remain, if then a sufficient degree of heat be applied to effect 

 their fusion, they leave upon the platinum spoon a permanent 

 light-blue stain. It would appear, therefore, that the metallic 

 oxides, which are always found in the ashes of wheat, exist 

 in the plant under an organized form, and are obtained by 

 incineration from this system of cups. And, hence, I con- 

 clude generally that earthy, saline, and metallic ingredients 

 enter as organizable products into the structure of plants. 

 Peckham, Oct. 2, 1837. 



LI V. On the Wave-surface in the Theory of Double Refraction. 



By J. W. Lubbock, Esq., F.R.S.f 

 QINCE Fresnel's discovery of the wave-surface published 

 ^ in his admirable paper on Double Refraction (Memoires 

 de V Institute tome vii.) much has been done to elucidate this 



* See the figure, Plate I. Phil. Mag., July 1837. 

 f Communicated bv the Author. 



Third Series. Vol. 11. No. 69. Nov. 1837. 3 H 



