TECHNOLOGY OF DIATOMS. 269 



of gum. This is how this water is prepared so as to have it 

 always ready. You choose some bits of Gum Arabic, extra white, 

 which are first washed and then dissolved in very pure distilled 

 water. This solution, which should be very fluid, is then filtered 

 twice through a double filter. You add to loo grammes of equally 

 pure distilled water some drops of the solution of gum. It is 

 needful that the quantity of gum so added shall be enough to fix 

 the Diatoms, and yet so thin that when dried it shall leave no 

 trace on the glass — that is to say, that the film deposited shall be 

 so thin as to be invisible. 



The covers, thus prepared and well dried, are ready to be 

 placed on the balsam. 



Cafiada Balsam. — A certain number of resinous media have 

 been, and still are, used for mounting Diatoms. First of all, 

 Venice turpentine was used, and then Canada balsam, which is 

 not really balsam, but simply a resin obtained from the Abies 

 Canadensis. As it can be obtained very pure and very limpid, it 

 is the medium most generally used in micrography. 



The best preparation of Canada balsam for general use is 

 obtained by allowing the balsam to thicken till it comes to the 

 consistence of a very thick mass. You then add enough benzole 

 to form it into a semi-fluid liquid. 



You place a drop of this balsam on the slip of glass, and then, 

 after having, by means of a pipette, applied a drop of benzole to 

 the Diatoms on the cover-glass, and quickly placed the latter on 

 the drop of balsam, causing it to be begin contact with the slip on 

 the left, and lowering it gently towards the right, using a mounted 

 needle. By this means you avoid the enclosure of air-bubbles, 

 which, though they may be got rid of by employing heat, are 

 nevertheless useless and troublesome. You then warm the slide 

 on a hot plate, or above a spirit-lamp, to expel the benzole and 

 harden the resin. The preparation, when cooled, is then washed 

 and cleaned with benzine ; then the cover-glass is sealed down 

 with a coat of cement of black lac, so as to guarantee the prepara- 

 tion from all external agents. As Canada balsam has only a feeble 

 index of refraction (i"52), it has long since been replaced, espe- 

 cially for the mounting of Diatoms, by Styrax and other similar 

 balsams. 



