104 GENERAL HISTORY OF THE INFrSORIA. 



tendency towards the light, at least, of many species, may be adopted by 

 placing the half-liquid mud in shallow pans or plates in the sunshine, when 

 many species may be found to rise as a film on the surface, or to congregate 

 near the edge or sides of the vessel. 



When the frastules are much intermixed mth mud, which is, under cer- 

 tain circumstances, inevitable, various plans have been adopted for separating 

 them for examination. Mr. Okeden details the following plan, which, with 

 certain modifications to be mentioned, has been described ako by Dr. H. 

 Munro : — 



" The plan " {J. M. S. 1855, pp. 158, 159) <' consists in making the deposits 

 fall through a constant depth of water, in various periods of time ; thus 

 dividing the Diatoms, according to their sizes, into portions of several dif- 

 ferent gravities." It is thus carried out : " Take about a cubic inch of the 

 clay to be examined, digest it for about four hours in strong nitric acid at a 

 moderate temperature ; now add gradually an equal quantity of hydrochloric 

 acid, effervescence takes place, a further action on the clay ensues ; keep 

 boiling for about three hours more, occasionally stirring, and then allow the 

 mixture to cool and settle do^vn, which it wiU do in about an hour ; pour off 

 the superfluous acid and wash the residue repeatedly with water, so as to get 

 rid of the remaining acid. 



" The next operation is to divide the sediment into portions of various 

 specific gravities : for this piu'pose it is necessary to have several beakers, 

 about 3 or 4 inches in height, and about 11- to 2 inches in diameter ; also 

 one very large beaker, about 6 to 9 inches in diameter : we wiU call the large 

 beaker A. 'Now transfer the sediment into one of the small beakers, and 

 pour in water tiU there is just 2 inches depth of water in the glass. Stir, 

 and let stand half a minute by the watch, and then pour off carefully into 

 the large beaker A ; repeat this about half a dozen times, each time pouring 

 off into A all that does not fall thi^ough the 2 inches of water in the half- 

 minute, and at last the small beaker wiU contain only what falls through 

 2 inches of water in half a minute. jSTow let A stand about half an hour, 

 pour off carefully, and transfer the sediment in A to another small beaker ; 

 put 2 inches of water with it, stir and let stand for 2^ minutes, then pour 

 off into A. Repeat this about six times, and there will now be another small 

 beaker containing all that falls through 2 inches of water in 2| miautes, 

 whUe in A is all that does not fall through that distance in that period. Let 

 A stand half an hour, pour off and transfer the sediment to another small 

 beaker, stir and let it stand ^ye minutes, pour ofi" into A as before, and repeat 

 this as before about six times. There is now another beaker, containing aU 

 that falls through 2 inches of water in 5 minutes. After this I do not di\ide 

 them any further, but call the last remainder, or what remains in A after 

 it has stood its half-hour, ' Kot in five minutes.' Thus we have four 

 different glasses, containing Diatoms and clay mixed, of four different densi- 

 ties : thus, to i ; -i- to 2-i- ; 2i to 5 ; not in 5. There is now a method of 

 concentrating the coarsest of these sediments, namely the to i, the i to 

 21-, and sometimes the 21- to 5. It consists in taking the beaker containing 

 the sediment and pouilng about an inch of water on it. Let it settle about 

 5 minutes, and then place the glass on a table, and impart a whirling motion 

 to the whole by moving it roimd and round, when the greatest portion of the 

 Diatoms will rise up in a sort of eddy, while the particles of mud or sand 

 will remain at the bottom, even though they are of the same specific gravity 

 as the Diatoms, and have fallen through the same distance of water in the 

 same time. This is because the Diatoms are mostly ^rt^ and thin, while the 

 particles of sand and mud are round ; in the same way, if we take a round 



