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PREPARATION OF DIATOMS. 79 



use as sizing," says Mr. Peragallo, " the gum-tragacanth recommended by 

 Professor Brun, of Geneva, and have always found it perfect ; its 

 refractive index being nearly the same as that of glass, renders its presence 

 invisible when the mounting is finished ; and moreover by using an 

 aqueous solution of it the following great advantage is secured, namely, 

 that all subsequent operations made with resinous media do not affect the 

 adherence of the diatoms to the glass, and that a type slide can be 

 prepared with heat in a few minutes without requiring either stove or 

 lengthy drying in the air, in other words, very little time, and scarcely 

 any apparatus are needed three solutions, a bristle needle, and a brass 

 heating table being the only requisites for making one or more completely 

 finished type preparations in the short space of five minutes. 



"This being so, for the sake of greater clearness I will give a 

 detailed description of my modus operandi step by step. 



"A. Solutions. ist. Solution of Styrax. Ordinary liquidambar dissolved 

 in benzine or in a mixture of benzine and absolute alcohol. 2nd. 

 Imbibing solution which should be the same as that which has been 

 used to dissolve the balsam ; a mixture of equal parts of benzine and 

 alcohol containing a little styrax dissolved in it is highly to be recom- 

 mended ; the styrax in the solution remains in the interior of the 

 diatoms and prevents the air from re-entering in case the liquid should 

 be by accident allowed to evaporate completely before applying the 

 styrax. 3rd. Fixing medium. This is obtained by making a saturated solution 

 of gum-tragacanth in warm distilled water and then filtering. The small 

 quantity of gum dissolved is amply sufficient ; a little alcohol or creosote 

 should be added to prevent mould. (This liquid has been recom- 

 mended by Professor Brun.) 



"B. Prepared Cover-glass. I fix or get fixed with styrax some 

 small cover-glasses of 5 mm. in diameter on some glass slips a little to 

 the side of the centre and on this cover-glass I deposit the selected 

 diatoms ; this arrangement is the characteristic feature of my method 

 and in my opinion offers the following advantages : 



" 1 st. I do not break or lose cover - glasses while manipulating ; 

 2nd. When I have deposited a diatom on a cover-glass, and have 

 fixed it there in the way I shall hereafter describe, I can write on the 

 slip observations which will enable me, if I have occasion, to 

 deposit subsequently another individual of the same species by the 

 side of the first ; 3rd. I can manipulate the cover-glasses which I am 

 using so as to keep them free from dust without either danger or risk, 

 lastly. When my preparation is finished, if I am not satisfied with it, 



