ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 343: 



the stiff glass plates are replaced by a pliant medium that allows fluids to 

 penetrate from both sides, the paraffin-imbedded sections being attached to 

 gummed paper by means of collodion. 



The gum-collodion plates are made by covering one side of a smooth 

 piece of writing-paper, duodecimo size, with a thick layer of gum arabic. 

 As soon as the gum ceases to be sticky, the paper is flattened out smooth 

 in a hand-press, care being taken that there are no uneveunesses. Thick- 

 flowing collodion is now passed over the gummy side, and a smooth layer 

 having been obtained, the plate is again pressed out between two firm, 

 smooth surfaces in the hand-press. Upon the plate thus obtained the sec- 

 tions are then fixed by means of a mixture of 2 parts collodion and 1 part 

 oil of cloves. For large and thin sections it is necessary to use a section- 

 stretcher of special construction. This, however, remains to be described. 

 Over the whole a thin layer of the collodion clove-oil mixture is then 

 brushed with a camel's-hair pencil. 



The paraffin is next dissolved out by laying the plate in a dishful of 

 benzin for 15-30 minutes. The plate is then dried with blotting-paper, 

 and at once transferred for a few minutes to 95 per cent, alcohol. It is 

 then dried again, and to keep up a perfectly smooth surface the sections 

 must be brushed over, if need be, with the collodion clove-oil mixture. It 

 is next transferred to 80 per cent, alcohol, wherein it remains for a quarter 

 of an hour or more. When sufficiently hard, these plates may be treated in 

 any watery or water-and-spirit solution. The watery solutions of course set 

 free the collodion plate by dissolving the gum. Clearing up and mounting 

 are performed in the usual way with creosote and Canada balsam. 



Permanent Caustic Potash Preparations.* — It is usually stated that 

 specimens treated with caustic potash cannot be permanently preserved. 

 During the past summer an aqueous solution of caustic potash of 85 to 40 

 per cent, was used by Prof. B. H. Gage for isolating cardiac muscle from 

 many different animals ; as some of the preparations were drawn it seemed 

 unfortunate not to be able to render them permanent as vouchers for the 

 drawings. This was accomplished by adding glacial acetic acid to the 

 isolated cells. The acid combines with the caustic potash to form acetate 

 of potash, which is often used for permanent mounting ; finally a mixture of 

 glycerin 75 parts and an aqueous solution of picrocarmine (1 per cent.) 

 25 parts was added as a permanent mounting medium. These specimens 

 after three months show nu signs of deterioration. If the specimens were 

 already under the cover-glass, a drop of glacial acetic acid was drawn under 

 it and afterwards a drop of the glycerin and picrocarmine mixture. 



How Alcohol drives out Air-bubbles, f — M. L. Errera remarks that 

 that which renders air-bubbles so persistent in organic tissues is, in the 

 first place, their extreme minuteness ; then the thin layer of water which 

 encompasses them holds in solution a certain quantity of organic matter ; 

 whence arise an increase of the superficial viscosity, and a diminution of 

 the tension, both favourable to persistence. 



But the air-bubbles should disappear if for water be substituted a liquid 

 endowed with the three following properties : — (1) It must be perfectly 

 miscible with water. (2) Its superficial tension must be weak. (3) Its 

 superficial viscosity must be weak. Now, of all the liquids indicated by 

 Plateau, who has made some very original observations on the superficial 

 and internal viscosity and tension of fluids, only two falfil both conditions. 

 These are ether and alcohol, both of which ought to rapidly drive out air- 



* The Microscope, vi. (1886) p. 267. 



t Bull. See. Belg. Micr., xiii. (1886) pp. 69-75. 



