74 PREPARATION OF DIATOMS. 



The liquid having cooled, the diatoms are washed and then boiled 

 in a solution of sodium carbonate of a strength depending upon the 

 circumstances of the case. This operation, which is carried out in a 

 porcelain dish, is carried on for ten minutes. 



After again cooling, the liquid is decanted and water, containing a 

 certain quantity of nitric or hydrochloric acid, is added. After again 

 decanting and washing, the deposit is poured into a long narrow flask, 

 so as to fill only a third of it and the whole is thoroughly shaken for one 

 or two minutes. The shaking separates the material ; water is then 

 added, after which it is allowed to rest for a tew minutes. The clear 

 portion is then decanted by means of a pipette and the deposit is again 

 shaken and the subsequent operations repeated, and so on as long as they 

 continue to give a good result, i.e., enable fresh diatoms to be separated. 



The mass which is ultimately obtained from these successive operations 

 is separated by decantation, if necessary, from any sand which may have 

 been left behind, and placed in a large flat-bottomed measure glass which 

 is filled up to the top and decanted into a measure No. 2 at the end 

 of a minute, this is again decanted into a measure No. 3 at the end of 

 three minutes, then again after five and ten minutes. The operation is 

 performed six times in succession, and timed by a watch, then the 

 diatoms will be sufficiently well separated according to their size. 

 The diatoms are then placed in a small test tube, and as much water 

 as possible is drained off, which should be replaced by pure alcohol. 

 This alcohol is then drained off again and twice replaced by isobutylic alcohol. 

 The diatoms, intended to be used for type-slides, are preserved permanently 

 in this isobutylic alcohol. When preparations are made the diatomiferous 

 liquid is placed, by means of a pipette, upon cover-glasses, which may then 

 be used either for immediate mounting or for the purpose of selection. 



The isobutylic alcohol cannot be mixed with water ; it has the advan- 

 tage of drying very slowly taking at least several hours and of avoiding 

 the accumulation of the diatoms during evaporation : the valves remain 

 absolutely in the same position in which they are placed at the moment 

 when the drop falls, but they do not stick to the cover-glass as is the 

 case with ordinary alcohol. 



Professor Brntis method of preparation. Professor Brun, the enthusiastic 

 diatomist of Geneva, described in 1887 a method of preparing diatoms, 

 which, according to experiments I have made, gives excellent results. He 

 has thus described his operations : 



" A large number of recent microscopical writers," says Mr. Brun 

 " have devoted their attention to pelagic gatherings, to those from lacustrine 



