78 Miscellaneous. 



On the Non-existence of Polnrizivq Silica in the Organic Kingdom*. 

 By Prof. J. \V. Bailey. 



It is now more than twenty years since Sir David Brewster an* 

 nounccd the existence ot" ptilaiizing or doubly refractive silica in the 

 cuticle of Eqiiisetinn, and in that of some of tlie grasses. In Lindley's 

 'Natural System of Botany,' tlie following account of Brewster's 

 experiments is given: — "On subjecting a f)ortion of tlie cuticle of 

 Equisetvm hyemule to the analysis of jiolarized light under a high 

 magnifying power, Brewster detected a beautiful arrangement of the 

 siliceous particles, which are distributed in two lines parallel to the 

 axis of tlic stem and extending over the whole surface. * * * * 

 Brewster also observed the remarkable fact that each particle has a 

 regidar axis of double refraction. In the straw and chaff of wheat, 

 barky, oats and rye, he noticed analogous phsenomena." 



In Quekett's 'Treatise on the Microscope,' 3rd edit. j). 3.38, direc- 

 tions are given for preparing the siliceous cuticle of Eqnisetum hye- 

 male for microscopic examination, by boiling in strong nitric acid, 

 and it is added that " in balsam it forms a beautiful object for polar- 

 ized light." Similar directions .ire given for preparing the silica in 

 the chaff of wheat, oats, &.c. 



As these statements arc contained in the last editions of each of 

 the above-mentioned works, it is evident that no contradiction of the 

 error involved in them has been pointed out ; yet, notwithstanding 

 the high authority on which they rest, the statements, so far as the 

 polarizing action of the silica is concerned, are wholly erroneous. If 

 the cuticle of the above-mentioned plants is completely deprived of 

 its carbonaceous tissues, it will be found wholly devoid of action on 

 polarized light, and any jireparation of the cuticle which is found to 

 affect polarized light, will also be found to blacken when heated in 

 concentrated sulphuric acid, and if then decarbonized by throwing 

 into the hot acid solution a little clilorate of potassa, the residual 

 silica shows no signs of action under the polarisi ope, either alone or 

 ■with the seleuite plate, although it still retains the forms of the cells, 

 stomata, &c. 



It is clear then that the error in the above statements has been 

 caused by the imperfect removal of the dense carbonaceous tissues 

 which are deposited beneath the silica. I have examined several 

 species oi Equisetum and a large number of plants of the Grass tribe 

 which are most remarkable for their siliceous cuticles, but have 

 found no trace of any action upon j)olarized light, when the carbo- 

 naceous matter was removed. But it is unnecessary to resort to arti- 

 ficial preparations to prove the correctness of my statements. Nature 

 has made her own preparations, and deposited them by myriads be- 

 neath every peat-bog, where may be found not only tlie siliceous 

 shells of the Diatoms and the spicules of the freshwater sponges, but 

 also a large number of the siliceous parts of the grasses, sedges, &c. 

 Ehrenberg has shown*, and I can confirm his statements, that the 



* Berlin Monthly Reports, May 1843. 



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