H. M0RLAND ON DIATOM STRUCTURE. 301 



and mounting ; I may say that it is not a case of " embarras de 

 richesses," far otherwise, and it is a question after all if it is worth 

 while to go through so much to obtain so little, more especially as 

 the diatoms show up very well in the sections of " Cementstein," 

 in addition to which sections of extreme tenuity can be found in 

 the "Cementstein," more particularly at the edges of the section, 

 which it would be impossible to lift on account of their really 

 being in pieces with the fragments still kept in position. 



Messieurs W. Prinz and Van Ermengem in their work on the 

 structure of diatoms contained in this " Cementstein," adopt 

 another method of isolating the diatoms ; they do not attempt to 

 select them, but merely immerse the finished section on the slip 

 in a weak solution of hydrochloric acid, which attacks and dis- 

 solves out the lime, still leaving the sliced diatoms attached by 

 the balsam to the slip. After washing gently in fresh water and 

 drying, they then mount in a watery solution of biniodide of 

 mercury and iodide of potash, which does not dissolve the thin 

 stratum of balsam by which the diatoms are fixed. This medium 

 has a high refractive index, 1*68, but this plan of mounting ne- 

 cessitates the diatoms being on the slip instead of on the cover. 



Having shown how diatoms in " Cementstein '' can be sliced 

 and isolated, perhaps some of our members may be able to extend 

 the process to other diatoms ; I have tried to do so myself, but as 

 I did not succeed to my satisfaction I need not trouble you with an 

 account of what I did and what I did not. If diatoms can be 

 cemented in bulk into a material having the texture and 

 characteristics of " Cementstein " the problem is easy. 



However, I do not think there is really any occasion whatever to 

 trouble about preparing sections of diatoms, as their structure can 

 be very well studied from broken valves, often found in much larger 

 quantities than desirable in the cleaned material. When selecting 

 diatoms I constantly come across broken valves or parts of valves 

 suitable for mounting for the study of their structure, and, pro- 

 vided that not more than one piece be required to be set " on 

 edge," I can generally place such piece in such position, adding 

 other pieces lying flat. I cannot now enter into particulars of 

 how I manage this, but could do so at length on some future 

 occasion. 



Supposing I have found a portion of a diatom valve, and have 

 set it so that the edge is in contact with the covering glass ; it 



