462 Dr. Wallich on the UMzopods. 



nitric acids " ! — Op. cit. p. 151. Indisputable proof being here 

 afforded by Prof. Leidy of these disks and plates being sili- 

 ceous, was it wise to assert that those in Dirfflugia symmetrica 

 are merely composed of chitinoid membrane ? 



It now only remains for me, before passing on from the 

 question of the generic status of Diffiugia symmetrica^ based 

 on the figure and composition of its test, to draw attention to 

 two or three collateral questions bearing upon what has gone 

 before. The first relates to a very interesting experiment 

 made by me with a view to find if rectangular plates resem- 

 bling those on its test could be produced artificially. The 

 experiment was eminently successful. It consisted in placing 

 in a suitable phial finely pulverized organic silica in the form 

 of '' tabasheer " (a substance well known as a product formed 

 within the joints of the bamboo) with an alkali and a mixture 

 of glycerine, gum, the albumen of ^gg^ potassium chlorate, 

 and, lastly, distilled water, the alkali in solution being kept 

 apart from the rest of the ingredients by a parchment dia- 

 phragm, and some cotton wool being placed loosely at the 

 iDOttom of the phial for any crystalline or other formation to 

 form or subside on. The phial and its contents were kept, at 

 the ordinary atmospheric temperature of summer, in a glazed 

 bookcase, where they could be examined without being in 

 anywise disturbed. After about five weeks, to my intense 

 satisfaction, distinct rectangular (square) plates were visible 

 with a powerful pocket-lens, having already attained a size 

 as large as the medium -sized plates usually found in Difflugia 

 symnip.trica, and in every respect resembling them. I would 

 add that I did not attempt to follow any precise quantitative 

 formula, but simply went by " the rule of thumb," looking 

 on the first effort as merely tentative and likely to give me 

 some clue to more accurate measurements on a future occa- 

 sion. I do not doubt, therefore, that any chemist would be 

 able to repeat the experiment with perfect success and with- 

 out the least difficulty. Of course I cannot but regard the 

 experiment as bearing in a very important degree on the 

 validity of my hypothesis regarding the colloidal metamorphism 

 observable in the plates and disks of the more marked forms, 

 and the chitinous pellets and cylinders observable on the tests 

 of the less highly metamorphosed forms, such as D. spiralis 

 and others. 



As regards Prof. Leidy's remark that the plates in Dijfflu- 

 qia symmetrica are not so symmetrically ai'ranged as the name 

 would imply, I have only to state that although I unques- 

 tionably employed the term to indicate the arrangement of the 

 plates in regular series, I also applied it to the perfect mathe- 

 matical figure of the plates themselves. 



