PREPARATION OF DIATOMS. 67 



By this operation the mass greatly increases in volume and the 

 organic matter becomes carbonized. The spirit lamp (or Bunsen burner), 

 which is being used, is then withdrawn, and, while the mass is still near 

 boiling point, a saturated solution of potassium chlorate is added drop 

 by drop to the water. As each drop is added a brisk effervescence is 

 produced and the liquid is stirred with a glass tube. When a certain 

 number of drops of potassium chlorate solution has been added the 

 quantity of which should be about half the volume of the sulphuric 

 acid used the liquid becomes quite clear ; the diatoms are then washed 

 as in the operation first described. 



It may sometimes happen that after this operation, the diatoms are still 

 not quite clean ; in this case the whole operation will have to be repeated. 



It is evident that very fragile diatoms and those but slightly silicified 

 could not stand so energetic a treatment ; it should therefore only be 

 used when the cleaning can be done in no other way. 



It should be again remarked that the last operation should only be 

 carried out in the open air or in a fume chamber on account ot the 

 chlorinated vapours, which are very dangerous to breathe, and moreover 

 because the solution of potassium chlorate can only be added drop by 

 drop without running the risk of a dangerous explosion and the projec- 

 tion of the matter out of the tube. 



The diatoms having been cleaned in this manner, it is then necessary to 

 prepare them. Various methods may be followed, but that which gives the 

 best results consists in burning the diatoms on a plate of mica or a 

 cover-glass. I have adopted this method ever since I was first 

 informed of it by the late Mr. De Brebisson, more than 25 years 

 ago. This process, together with the preliminary and subsequent opera- 

 tions, has been described at full length by my friend Professor H. L. 

 Smith, the learned American, so well known by his many excellent 

 works on diatoms. I quote the article which he published on this 

 subject in "The Lens," the journal of the State Microscopical Society 

 of Illinois. (') 



Preparation of Diatoms according to Prof. H. L. Smith. -The follow- 

 ing article, in the words of Prof. Smith, deals with the rapid method of 

 preparing from crude material and with a mode of mounting invariably 

 on the cover of the slide : 



t; The gatherings should not be dried, but kept moist, in phials with 

 a little creosote to prevent mould. I very much prefer to examine 

 whole frustules, with both valves adherent, or if filamentous, still 



( ) The Lens (1873), ii., pp. 209-212. 



