Miscellaneous. 315 



within the margins of the covering-plate. The latter is now to be 

 placed upon the specimen, and its margins finally covered with the 

 cement. If there is too much fluid beneath, the superfluity finds a 

 channel for escape ; an opening then takes place in the cement, below 

 the cover, but is again closed, if care be taken to renew the application 

 of the cement to the edges of the cover, when the superfluous fluid 

 has been removed, or has dried up. In about two days, the outer 

 layer of the luting will have become dry, but the inner layer remains 

 soft for many weeks and even months. This is just what constitutes the 

 excellence of the cement, for it never bursts and permits evaporation ; 

 and a great number of preparations which I have put up in this 

 manner are at the present time, after the lapse of several years, quite 

 unaltered. It is, however, of importance that the cement shall occupy 

 a portion of the space between the object-plate and its cover ; a mere 

 anointing of the edges of the latter is never sufficient. 



If the specimen be one which will not bear pressure without injury, 

 it must be put up in some kind of cell, the depth of which must be 

 regulated by the thickness of the object. The covering-plate must 

 in this case be always smaller in diameter than the space between the 

 outer margins of the cell. First, some preservative solution is placed 

 in the cell, and then the object is laid in it ; the upper edges of the 

 cell are then touched with a little of the gtUta-percha luting*. The 

 cell is then completely filled till the fluid even forms a convexity above 

 its margins ; if now the cover is applied, the superfluous moisture 

 escapes, and no air remains in the cell. Finally, when the edges are 

 dry, they must be covered with a thick layer of the luting, and with 

 a second a few days afterwards. 



The method last described is especially applicable to the preserva- 

 tion of injected specimens in a solution of arsenious acid. — Vol. ii. 

 p. 350-355. 



Preparation of Caoutchouc Cells. 



In commerce we now obtain caoutchouc plates of difi^erent thick- 

 ses. The thinnest measure about one millimetre {-^th. of an 

 inch), and out of these plates of any required thickness may be 

 formed, as their surfaces adhere perfectly together, especially if pre- 

 viously slightly heated. In a square piece of suitable thickness an 

 opening may be made by means of scissors, or the centre may be 

 cut out of a disc-shaped piece by means of a hammer and ring-shaped 

 punch. To fasten the caoutchouc ring to the object-slide we use the 

 following luting : — 



One part of finely cut gutta-percha is mixed with fifteen parts of 

 of turpentine, and dissolved in it by gently heating, and constantly 

 stirring, the mixture. The solution is then poured through a cloth, 

 to separate some impurities which are always to be met with in raw 

 gutta-percha. To the purified solution there is added one part of 

 shell-lac, which, by the aid of gentle heat and constant stirring, must 

 be dissolved in it. The heat is then kept up until a drop of the solu- 



* The reader will find the receipt for this composition, and directions for 

 king cells of gutta-percha and caoutchouc, at tlie end of the jwesent 

 icle. — Tkan.s. 



v=m fol 



II. 



