?6 HOW TO WORK 



napJitJia and water, or carbolic acid and water. If we require a fluid 

 of higher specific gravity, some of the saline solutions, diluted with 

 a proper quantity of water, may be used. If we wish for a solution 

 which refracts highly, we may employ glycerine, or a mixture of 

 glycerine and gelatine ; while, if we require a fluid which has the pro- 

 perty of hardening the structure, we may immerse it in a solution of 

 chromic acid, bichromate of potash, corrosive sublimate, or diluted alcohol. 



In all cases the substance shot/Id be immersed for some time in (lie 

 fluid, in which it is to be preserved, before being mounted permanently. 

 The cell made of Brunswick black or the thin glass cell, or other forms 

 described in 1 16 131, may be chosen according to the dimensions 

 of the specimen. The object and fluid being placed in the cell, the 

 thin glass cover is applied, with the precautions to which I shall 

 presently advert. The superfluous fluid is removed with a piece of 

 blotting paper, or a soft cloth, and after the edges have been allowed 

 to dry a little, they are anointed with a thin layer of Brunswick 

 black. 



Almost every organised structure, and especially the soft moist 

 tissues of the bodies of animals, may be advantageously preserved in 

 fluid. It has been said that the solution employed in preserving a 

 structure should resemble as nearly as possible in density and refrac- 

 tive power, the fluid which bathed it during life, but it is a fact that 

 many even exceedingly delicate structures may be examined in fluids 

 of high density, as syrup or glycerine, and peculiarities may be made 

 out which are not to be demonstrated when they are examined in 

 water or serum. 



143. Examination in Canada Balsam, Turpentine, and Oil. The 

 well-known Canada balsam has long been a favourite medium for the 

 preservation of microscopical specimens, on account of its pene- 

 trating and highly refracting powers. Turpentine possesses very 

 similar properties, but from being a limpid fluid, it is far less useful 

 than Canada balsam. All preparations to be mounted in Canada 

 balsam must be thoroughly dried first. The desiccation must be 

 effected by a temperature of not more than from 100 to 200 degrees. 

 For the purpose of drying tissues, we may employ the water-bath 

 alluded to in 73, or we may place the specimen under a bell-jar close 

 to a basin of strong sulphuric acid or chloride of calcium, which sub- 

 stances have the power of absorbing moisture in an eminent degree, 

 pi. XX, fig. 131. Many textures in process of drying include a 

 number of air bubbles in their, interstices, and it is often very difficult 

 to remove these. To effect this object, the preparation may be 

 allowed to soak some time in turpentine, and the removal of the air 

 is often much facilitated by the application of a gentle heat. If the 



