266 



TECHNOLOGY OF DIATOMS. 



N\S 



after the second time, a little sulphuric acid is added, which neu- 

 tralises the soda that remains and completely cleans the glasses. 

 The least trace of acid is then removed by very pure distilled 

 water, and the glasses are allowed to dry spontaneously, screened 

 from dust ; or, better still, they can be dried near a stove. There 

 then remains nothing but to wipe them before using. 



The glasses, being wiped, are placed at some dis- 

 tance from each other on a plate of metal or very level 

 glass. You then take, by means of a pipette (armed 

 with an elastic ball), Fig. i8, a certain quantity of 

 liquid containing Diatoms in suspension, and these 

 you have well mixed up with the pipette ; then, whilst 

 with one hand you hold the glass lightly with a 

 mounted needle, you cover it with the liquid by lightly 

 pressing the india-rubber ball. If the quantity spread 

 appears too great, you can withdraw the desired quan- 

 tity by reducing the pressure. A little practice will 

 put you in the way of all the little details of this 

 operation. 



If the glasses are as they should be, the water will 



spread very evenly up to the edge. It will present a flat 



surface, never in bulges. Nothing now remains but to 



allow them to dry spontaneously, screened from dust, 



or dried by a stove, at a temperature of 4o^C. A too- 



Fig. 18. brisk dessication, or the use of too high a temperature, 



generally sets up currents, which cause the Diatoms to group 



themselves in little masses of a very annoying character. 



Burning Diatoms on the Platinum Plate. 



Certain gatherings, such as contain delicate kinds — especially 

 marine species, which do not admit of the use of acids during the 

 process of cleaning (almost all the marine gatherings are of this 

 class), and which have only been treated with distilled water and 

 the " tamisage " — ought to be at once spread on the glass and then 

 treated to a dull red heat on a plate of platinum, so as to destroy 

 the organic matters, which are found independent of the endo- 

 chrome in which each frustule is contained, and of which the 

 presence, now useless, tends to obscure the details of the siliceous 

 carapace. 



