TECHNOLOGY OF DIATOMS. 353 



The composition of those most commonly used is as follows : 

 Gum-lac Cement.— Dissolve a small quantity of white lac in 

 alcohol of 90 per cent., making the solution of such strength that 

 a drop spread on a slip, slightly warmed, leaves a film of lac very 

 thin and transparent. This solution ought to be filtered after 

 resting, so that the insoluble and flocculose portions may subside. 

 As the solution is alcoholic, it spreads spontaneously and evenly 

 over the slip. It is sufficient to place in the centre a small drop 

 by means of a small glass pipette ; the only precaution that is 

 absolutely necessary to be taken, if you wish to get an even and 

 transparent surface, is slightly to warm the slips before putting the 

 cement on them. 



Gelatine and Fish-Glue Cements.— These two cements are very 

 good, especially the second ; the preparation is as follows : — 

 Russian Isinglass, or extra-white Gelatine- i gramme. 

 Phenique, Distilled Water, as i to 100 - 100 ,, 

 Acetic Acid (crystallisable) - - - i „ 

 Steep the gelatine or isinglass for an hour or two in the water ; 

 then dissolve by the application of heat; filter, while hot, through 

 a simple filter-paper, and then a second time, while still hot, 

 through a double or triple filter ; then add the acetic acid, the 

 object of which is to make the solution quite clear and to main- 

 tain it in a liquid state. 



Resinous Cements. — The solutions already described on pp. 

 268 — 271 can be used with equal success. 



For Fixing the Diatoms on the lac, it is necessary to slightly 

 warm the slips on which they are placed ; for the other three, it 

 will be enough to breathe softly on them. 



For the more convenient preparing of the cover-glasses, I fix 

 them temporarily on a slip by breathing strongly on it and then 

 rapidly slipping the cover-glass on this humid surface. Nine times 

 out of ten the cover-glass adheres sufficiently to allow of its being 

 perfectly cleaned ; it is covered with the fixing liquid, and the slip 

 serves to support it while you are placing the objects upon it ; 

 after which a slight exposure to the flame of a spirit-lamp will 

 separate the cover-glass from its support. 



