PRESERVATIVE MEDIA. 223 



again, recourse may be advantageously had to Goadby's Solution, 

 which, is made by dissolving 4 oz. of Bay-salt, 2 oz. of Alum, and 4 

 grains of Corrosive Sublimate, in 4 pints of boiling water : this 

 should be carefully filtered before it is used ; and for all delicate 

 preparations it should be diluted with an equal bulk, or even with 

 twice its bulk, of water. This solution must not be used where 

 any Calcareous texture, such as Shell or Bone, forms part of the 

 preparation ; and one of Mr. Goadby's other solutions (8 oz. of bay- 

 salt and 2 grs. of corrosive sublimate, to a quart of water, — or, 

 in cases where the coagulating action of Corrosive Sublimate on 

 Albuminous matters would be an objection, the substitution of 20 

 grains of Arsenious acid, ) may be used in its stead. Preparations 

 of the Animal Tissues to be examined as transparent objects under 

 high magnifying powers, may usually be advantageously mounted 

 either in Farrants's medium or in Glycerine -jelly. Carbolic Acid 

 has recently been employed as a preservative medium ; but the 

 Author has had no experience of its use. — It is often quite impos- 

 sible to predicate beforehand what Preservative Medium will answer 

 best for a particular kind of preparation ; and it is consequently 

 desirable, where there is no lack of material, always to mount the 

 same object in two or three different ways, marking on each slide 

 the method employed, and comparing the specimens from time to 

 time, so as to judge how each is affected. It may be stated, how- 

 ever, as a general rule, that objects to be viewed by light reflected 

 from their surfaces should not be mounted in either of the Gela- 

 tinous media, but in Diluted Alcohol, Goadby's Solution, or some 

 other liquid which does not tend to render them transparent. 

 Objects mounted in Gelatinous media, on the other hand, are often 

 shown admirably by Black-ground Illumination (§ 84). 



164. Of Mounting Objects in Fluid. — As a general rule, it is 

 desirable that objects which are to be mounted in fluid should be 

 soaked in the particular fluid to be employed, for some little time 

 before mounting; since, if this precaution be not taken, air- 

 bubbles are very apt to present themselves. It is sometimes 

 necessary, in order to secure the displacement of air contained in 

 the specimen, to employ the Air-pump in the mode already 

 directed (§ 160) ; but it will generally be found sufficient to im- 

 merse the specimen for a few minutes in Alcohol (provided that 

 this does not do any detriment to its tissues), which will often 

 penetrate where water will not make its way ; and when the 

 spirit has driven out the air, the specimen may be removed back 

 to water, which will gradually displace the spirit. When Deane's 

 Gelatine or Glycerine-jelly is used, however, all that can be done 

 will be to drain the object of superfluous water before applying 

 the liquefied medium ; but as air-bubbles are extremely apt to 

 arise, they must be removed by means of the Air-pump, the Gela- 

 tine being kept in a liquid state by the use of a vessel of hot 

 water, as in the case of Canada balsam. In dealing with the 



