or THE DIATOME^. 105 



pebble and an oyster- shell both of the same weight, and throw both hori- 

 zontally into the water, the pebble vnll reach the bottom sooner than the 

 oyster-shell. 80, when the whirling motion is imparted to the glass, the thin 

 flat shells of the Diatoms will rise up in a cloud, while the round particles of 

 mud and sand will remain behind ; when the cloud rises up, pour it off quickly 

 and dextrously into another glass, and, if necessaiy, repeat the process ; and 

 a little practice will enable the operator to separate all the Diatoms most 

 effectually. I have said before that this process will only apply to the to ^, 

 ^ to 2i, and sometimes the 2^ to 5 sediment, but not to any finer one ; 

 practice will soon teach this. The ' not in 5 ' cannot be concentrated — it is 

 too fine, and the whole rises together on imparting the whirling motion to it. 



" It is not necessaiy to abide invariably by the divisions of time which I 

 have given here. 



" These must be varied, of coui-se, according to the nature of the clay to be 

 examined. For instance, in a clay I have recently tried from 34 feet below 

 the bed of the river at Cardiff, nearly the whole of what was left after the 

 to i fell in the ^ to 2^. I therefore divided it thus : to i, -i- to li, and 

 li to 2^ ; a little practice wiU. soon teach this. 



" The advantages of the plan are, I think, ob\'ious. In the fu^st or coarsest 

 sediments we get all the larger and finer Diatoms by themselves, unmixed 

 "svith, and consequently unobscui'ed by, the innumerable smaller ones and 

 the fine particles of mud and sand, while, if any of them, such as the Eupo- 

 disci or Cmnpylodisci, are rare, they are sure to be found in either the first or 

 second di\dsion of densities, and by their being concentrated and brought as 

 it were into a small compass, the detection of them is easy and certain. 



" In the next division, or the 2^ to 5, we shall find the moderate-sized 

 Diatoms ; and lastly, in the ' not in 5,' we get a mass of the remaining and 

 smaller Diatoms, all of which small ones are themselves the more readily seen 

 and identified when separated from their larger brethren. 



" I would ventui'e to add, moreover, that I think the examination of these 

 deposits for the various species is much facilitated, as the slides containing 

 the to li sediment may be examined with the inch objective, the -J-inch 

 will do to examine the 1^ to 2^ and 2^ to 5, while the -i-inch need not be 

 used till we come to the ' not in 5 ; ' whereas, were they all mixed, the i-inch 

 would be required to examine the whole. 



" I should add, that what is poui-ed off the large beaker A, after it has 

 stood the half-hour each time, may be fiimg away and the sediment only 

 transfeiTed to the small beakers, as from the large size of it there T\all rarely 

 be more than 2 inches depth of water in it, and half-an-hour is ample time 

 to ensure every diatomaceous particle falling to the bottom and being pre- 

 served and detected in one or the other of the divisions." 



Dr. Munro's plan is a variation of the above proceeding, and is thus de- 

 tailed {J. M. S. 1855, p. 242) : '' I first boO. 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 possible ; then treat the deposit 

 in the same way T\ith strong nitric acid, washing the deposit by repeated 

 washings to get rid of the remaining acid. When this is done, I then sepa- 

 rate 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 fom^ 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 duiing any stage of the process. Having nearly filled this tube 

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

 watch the deposit as it falls sloAvly and gradually down the tube, and with a 



