314 Miscellaneous. 



VII. Solution of Ar senile of Potass. 



I have, in a few instances, made use of a solution of arsenite ol 

 potass in 1 60 parts of water, to preserve the primitive nerve-tubes. It 

 has been found as effectual as the carbonate of potash solution. 



In employing the chloride of calcium solution * and Canada Balsam, 

 it is unnecessary to take measures to prevent the evaporation of the 

 fluid. The first remains always fluid, — chloride of calcium being a 

 deliquescent salt ; and as the outer surface of the balsam hardens, the 

 escape of the liquid portion is prevented. 



But it is otherwise with the last-mentioned preservative fluids 

 (Nos. III. to VII.). To prevent their evaporation, it is necessary to 

 employ a cement or luting to prevent air from having access to the 

 fluid. Different compositions have been recommended for this pur- 

 pose ; but I have found none more serviceable than that employed by 

 gilders to make gold-leaf adhere to mirror and picture frames. The 

 following is the receipt for the preparation of this so-called gold- 

 ground or gold-size : — 



Let twenty-five parts of linseed oil be boiled for three hours with 

 one part of red lead (menie) and one-third of a part of umber, and 

 then poured off. Next take white lead and yellow ochre, well pounded 

 and divided, and mix them together in equal proportions. Successive 

 portions of this mixture must be added to the oil, and well rubbed up 

 and mixed with it, till a tolerably thick fluid is formed, which must 

 be once more thoroughly boiled. 



If now a preparation has been made, which it is wished to preserve 

 in the chloride of calcium, or any of the five last-mentioned fluids, 

 and if it can, without injury, bear a little pressure, the following 

 manipulation is recommended : — 



If the specimen is moistened with water, which during the preli- 

 minary examination is frequently the case, all superfluous fluid is in 

 the first place removed with a little roll of bibulous paper, or with a 

 camel-hair pencil, such as I have elsewhere recommended. The 

 fluid at a little distance from the object may be wiped off with a cotton 

 or linen rag, and the surface of the glass there made perfectly dry. 

 A certain quantity of the preservative fluid is then placed upon the 

 specimen, and this is most conveniently effected by using a dropping- 

 flask. The amount of fluid should be such that it should afterwards 

 perfectly fill the space beneath the covering plate ; the proper 

 quantity is soon learned by a few trials. Next a (square ?) covering- 

 plate, about two millimetres (^^th of an inch) narrower than the 

 object-slide, should be laid under the centre of the latter, — i. e. im- 

 mediately beneath the part which it is destined to cover. A pencil 

 is next dipped in the cement, and a square drawn with it upon the 

 glass around the fluid containing the specimen, so that the cement 

 shall extend from one to two millimetres ( aV'^ ^^ tV^^ ^^ ^^ inch) 



* The author has renounced the practice of putting up preparations in 

 this fluid, and permitting the access of air, for reasons given at p. 312. 

 — Trans. 



