CHEMICAL TESTING PRESERVATIVE MEDIA 517 



as follows : Evaporate 100 c.c. of liquor zinci chloridi (B.P.) to 

 70 c.c. ; dissolve in it 10 grins, of iodide of potassium ; then add 

 0'2 grin, iodine ; shake at intervals till saturated. 



This is extremely useful for the detection of pure cellulose. The 

 zinc chloride converts cellulose into amyloid, which is then turned 

 blue by free iodine. Wood-cells, cork-cells, the extine of pollen 

 grains, and all lignified or corky membranes, are coloured yellow. 

 Starch colours blue, but is rapidly disorganised. 



A very weak solution will instantly detect tannin, the cell con- 

 tents in which it forms a part becoming reddish or violet. 



y. Solution of caustic potass or soda (the latter being generally 

 preferable) has a remarkable solvent effect upon many organic sub- 

 stances, both animal and vegetable, and is extremely useful in 

 rendering some structures transparent, whilst others are brought 

 into view, its special action being upon horny textures. whose 

 component cells are thus rendered more clearly distinguishable. 



8. Dilute sulphuric acid (one of acid to two or three parts of 

 \\atcr) gives to cellulose that has been previously dyed with iodine 

 a blue or purple hue ; also, w r hen mixed with a solution of sugar, it 

 gives a rose-red hue, more or less deep, with nitrogenous substances 

 and with bile (Pettenkofer's test). 



Sulphuric acid causes starch grains to swell and similarly affects 

 cellulose. 



. Concentrated nitric acid gives to albuminous substances an 

 intense yellow. 



. Acid nitrate of mercury (Millon's test) (ten parts of mercury, 

 ten of fuming nitric acid, and twenty of water) colours albuminous 

 substances red. 



77. Acetic acid, which should be kept both concentrated and diluted 

 with from three to five parts of water, is very useful to the animal 

 histologist from its power of dissolving, or at least of reducing to such a 

 stage of transparence that they can no longer be distinguished, certain 

 kinds of membranous and fibrous tissues, so that other parts (especially 

 nuclei) are brought more strongly into view. 



6. Ether dissolves resins, fats, and oils ; but it will not act on 

 these through membranes penetrated with watery fluid. For tlie 

 same purpose chloroform, benzol, oil of turpentine, and carbon bisul- 

 phide are used. 



i. Alcohol dissolves resins and some volatile oils, but it does not 

 act on ordinary oils and fats. It coagulates all luminous matters, and 

 consequently renders more opaque such textures as contain them. 



K. Osmic acid is a test for fatty matters, which it stains black- 

 in varying degrees ; and in like manner for gallic and tannic acids. 



Preservative and Mounting Media. We have now to consider 

 the various modes of preserving the preparations that have been 

 made by the several methods indicated above, and shall first treat 

 of such as are applicable to those minute animal and vegetable 

 organisms, and to those sections or dissections of large structures, 

 which are suitable for being mounted as transparent objects. A 

 broad distinction may be in the first place laid down between 

 resinous and aqueous preservative media ; to the former belong 



