242 MEMORANDA. 



and having lately used a similar process with some success, by 

 allowing the Diatoms to fall through a given length of water, 

 I beg to forward you the method I have adopted. 



I first boil the deposit in strong hydrochloric acid for five 

 or ten minutes, then allow it to subside, pour off all the 

 acid, and by a few washings get as much of it away as pos- 

 sible. Then treat the deposit in the same way with strong 

 nitric acid, washing the deposit by repeated washings to get 

 rid of the remaining acid. When this is done, I then separate 

 the Diatoms according to their different gravities by allowing 

 them to pass through a column of water in the following 

 manner : — 



I take a long glass tube about four feet long and half an 

 inch in bore. At the bottom of this tube is fixed a stop-cock 

 to enable me to let out any of the Diatoms during any stage 

 of the process. Having nearly filled this tube with distilled 

 water, I pour in my deposit washed free from the acids. I 

 watch the deposit as it falls slowly and gradually down the 

 tube, and with a Codington lens can easily detect the larger 

 Diatoms as they are precipitated. In about a quarter of an 

 hour, many of the larger forms will have descended to the 

 bottom of the tube. By turning the tap at the bottom of the 

 tube, I let out a drop of the mixture on a slide, and examine 

 it with a. low power (J-inch) ; and if it be tolerably clear, and 

 the Diatoms of one character, I then let off five or six inches 

 of the mixture into a test-tube, and set it aside for re- 

 examination after the Diatoms have subsided. In a quarter 

 of an hour more, I again let off into another test-tube six 

 or eight inches more of the mixture, and place it aside to 

 settle. In half an hour more I let off into another test-tube 

 six or eight inches of the mixture, which will contain the finer 

 Diatoms by themselves, generally free from all mud and 

 sand. I then pass each of these washings again through 

 the long tube of distilled water ; and by examining the 

 mixture during the process of its subsidence, I am enabled 

 to let out the heavier particles of sand or mud, and to obtain 

 pretty clean all those Diatoms which are alike in size, or at all 

 events in specific gravity. Some Diatoms take a longer time 

 than others in settling to the bottom of the tube, and separat- 

 ing themselves from extraneous matter, such as the Nitzscliia 

 closterium, &c. ; but, by a little patience, and an extra washing 

 through the tube, these difficulties may, in a great measure, 

 be overcome. By this method, I have found the Pleiirosigmata, 

 PinnularicB, SuinrellcB^ and Synedrce, very well separated, those 

 of a like character being found together. 1 have been 

 stimulated to send you these few remarks on the washing of 



