TECHNOLOGY OF DIATOMS. 



267 



.V^ 





/. 



19- 



The small plate of platinum that is needful for this purpose 

 should be neither too thin nor too thick. If 

 too thin, it will present superficial irregulari- 

 ties, which will prevent the glasses from so 

 resting on it that they may be altered in shape 

 under the influence of the heat if it should be 

 a little too great ; and if too thick, it will cost 

 more than is needful without corresponding 

 advantage. That which I use measures 

 35 X 30 mm., and has a thickness of from 

 i/ioth to i^. I hold it between the jaws of 

 a little test-tube holder (Fig. 19), to expose it 

 to the flame of a small spirit-lamp. 



Three or four glasses of 1 6 mm. may be 

 burnt at the same time on a platinum plate of 

 the size I have indicated. It is placed above 

 the flame and gradually lowered until it 

 assumes a dull-red tint ; you may in certain 

 cases carry it as far as a cherry red, but with 

 great care and for a very short time. In order 

 to judge the better of the intensity of the heat 

 as represented by the colour that the platinum 

 plate assumes, it is well to conduct the opera- 

 tion in a feeble light. During the operation 

 the surface of the glasses becomes, first, 

 brown, then black, and then perfectly white, 

 which indicates that the carbonaceous matter 

 is all consumed, and it only remains to fix 

 the glasses on the cells that have been pre- 

 pared for them. 



The glass slip, with the cell, having been 

 wiped from dust, is slightly warmed over a 

 spirit-lamp to drive off any moisture; or, 

 better still, placed on a plate having a tem- 

 perature of about 4o''C. You place the thin 

 glass with the diatoms downwards on the cell, 

 and, holding the slip by one end, warm the other in the lamp- 

 flame, pressing lightly and perpendicularly on the cover-glass with 



