216 



HARDWICKE'S SCIENCE-GOSSIP. 



[Nov.], ] 866. 



After stating his opinion that the preservative 

 liquid recommended by Dr. Schacht for vegetable 

 preparations is not sufficiently concentrated, M. 

 Belleroche advocates a mixture of one part of 

 chloride of calcium to three of distilled water, as 

 not altering the colours of such preparations, except 

 with desmids (which it changes to a delicate olive) 

 and then proceeds : — 



" In the method I employ to make my vegetable 

 preparations (using always a square cover for the 

 object), I make use of no other instruments besides 

 a pair of metal forceps, so as not to touch the cover 

 with the fingers after having cleaned it ; a scalpel 

 extremely thin at the point, to raise the cover if 

 occasion should arise ; a brash, sufficiently fine to 

 use with the cement; and some 

 blotting-paper to remove the ex- 

 cess of liquid from the glass slip. 



"I trace upon the slide with 

 the black varnish, two parallel 

 bands in the direction of its length, 

 somewhat longer than the cover, 

 having consideration also as to its 

 width, as represented in fig. 228. 



" You will observe, Sir, that I 

 leave the two ends open. The 

 thickness of the object to be pre- 

 pared determines the number of 

 coats of varnish to be superposed 

 in order to get the requisite depth ; 

 it is but rarely that this exceeds 

 three layers, which have sufficed 

 with me for the greater number of 

 mosses, &c. A greater thickness 

 would exclude the employment of 

 certain objectives, requisite how- 

 ever for details of objects which 

 require, as a whole, only a low 

 magnifying power. Finally, this 

 prevents the closing of the two 



Fig. 22S. 



thickness never 



ends after putting on the cover. 



" Before using the cover I moisten the surface of 

 the cement with a last and very thin coat of the 

 same substance. To insure adhesion I place between 

 the two bands a pretty large drop of (the preser- 

 vative) liquid, next the object to be prepared, giving 

 the latter time to soak ; then I let the cover fall 

 flat upon the liquid, having found that this is the 

 best method for driving out the air which might 

 otherwise remain in the preparation. The cover 

 being fixed by the requisite pressure, I add some of 

 the liquid (if necessary) by capillary attraction, and 

 if the liquid overflows at either or both of the ends, I 

 remove the excess by means of blotting-paper. The 

 liquid being perfectly concentrated between the two 

 glasses, and the surrounding part properly dried, I 

 close up the two ends with the brush, and finish the 

 first closing-in {encadrement) with the cement ; and 

 on the following day, when this coating is dry, I 



apply a second to it, to complete the operation, 

 giving to the cement a thickness sufficient to make 

 it level throughout, and to finish the edges neatly. 



" If the operation has been properly performed 

 there will be no air; but if, by too much precipita- 

 tion, it has been badly done, and if there remains 

 a little air, the cover should be gently raised with 

 the aid of the scalpel at the side nearest the bubble, 

 and after having liberated the air, as much liquid 

 should be added as it will absorb, but on the opposite 

 side, so as to avoid the chance of producing other 

 bubbles by shutting the air in. It will be under- 

 stood that in this case it will be necessary to exercise 

 a fresh pressure, and again have recourse to the 

 use of the blotting-paper. 



" Glycerine, treated in the same manner, however, 

 offers a little more difficulty, for that which runs 

 over on the slide is not so easily removed as the 

 calcium, and unless it is completely dried up, the 

 cement does not adhere to the glass. 



-3? 3- Vf % '5iC- 



" It is not always an easy matter to complete the 

 quantity of liquid between the two glasses by capil- 

 lary attraction. If as the result of the preliminary 

 operation it is no longer to be feared that the ob- 

 ject, already immersed in the liquid, is in immediate 

 contact with air bubbles, it is none the less possible 

 that during the infiltration blank spaces may be 

 formed around the object, a diffi- 

 culty which necessitates the rais- 

 ing of one end of the cover, which 

 we should always endeavour to 

 avoid. We succeed by taking care 

 that the liquid applied for absorp- 

 tion be in very small quantity at 

 a time, and by frequent repetition, 

 and that the application be made 

 at any point of the two extremi- 

 ties, where the liquid touches the 

 edge of the square glass. My 

 sketch (fig. 229) will give a correct 

 idea of this. 



" The cover has just been put on, 

 and the pressure necessary to in- 

 sure adhesion exercised ; the ends 

 C and D are still open ; B repre- 

 sents the void which remains to 

 be filled up ; A is the liquid by 

 which the object is perfectly sur- 

 rounded, it touches the edge at 

 C. It is there then that we must 

 place successively, and until filled up, the drops of 

 the liquid to be absorbed. 



"Success is much more certain if, before applying 

 the cover, care is taken to spread the liquid in the 

 middle between the two bands of cement, so that it 



may touch them It is a very important point 



to give as much liquid as the glasses will absorb, and 

 to allow sufficient time for perfect absorption, even 



Fig. 229. 



