V 



J. DEBY ON THE STRUCTURE OF THE DIATOM VALVE. 311 



the solution is to take place by cementing a bit of mica to a glass 

 slide, and then cover all its surface except a central cell with wax. 



" In this cell the shells are put with a little water, and after 

 adding a drop or two of the acid by means of a dropping rod of 

 silver or platinum, the cell is covered with another plate of mica, 

 and the action watched under the microscope. 



" If hydrofluoric acid is applied to recent Diatomacea?, the silica 

 soon dissolves leaving distinct, internal, flexible cell-membranes 

 retaining the general form of the shells. These may sometimes, 

 but not generally, be detected even in the fossil specimens. 



" When present they materially interfere with the examination 

 of the true nature of the markings of the siliceous shell, and 

 should be destroyed by nitric acid and heat before the hydro- 

 fluoric acid is employed, unless it is desired to study the cell- 

 membrane itself. There is a curious difference in the action of 

 hydrofluoric acid of the same strength upon specimens of Fossil 

 Diatomacea3 from different localities. Some dissolve with even 

 too great rapidity in an acid which is slow and tedious in its action 

 on other specimens. The Bermuda and Richmond Tripoli, and 

 some specimens of fluviatile origin, resist the action much longer 

 than is usual with most specimens, whether they are recent marine, 

 or either recent or fossil fluviatile ones. This difference is prob- 

 ably due to different degrees of hydration." 



III. 



My direct observations have been made on a series of special type- 

 slides, all of which are open to inspection to an} r persons interested 

 in the subject. These slides were mostly prepared for me by Mr. 

 E. Thum, of Leipzig, with unparalleled care, patience, and 

 dexterity, and I am highly indebted to him for my very best 

 material and for many very remarkable diatoms, mounted in various 

 media, and showing structure better than any I possessed before. 



All critical cases I have examined under dry objectives, water- 

 immersion glasses, and the homogenous lenses in my possession, 

 namely : one-twenty-fifth and one-sixteenth horn, imm., by Powell 

 and Lealand ; one-tenth and one-fourth horn, imm., by Spencer, 

 of Geneva, N.Y. ; and one-eighth horn, imm., by Zeiss ; all glasses 

 of most recent make and of very great excellence. 



Every diatom, or fragment of a diatom, has been examined by 

 me both by transmitted and by reflected light, by direct and 



