14 PREPARATION AND MOUNTING 
impossible in any other mediwm. All that is required is, 
that the strength of the fluid should be increased very: 
gradually until the whole tissue is thoroughly penetrated by 
the strongest that can be obtained;” and “ that thus very 
hard textwres may be softened, so that by gradually increas- 
ing pressure and careful manipulation exceedingly thin 
layers can be obtained, without the relation of the anatomi- 
cal elements to each other being much altered, or any of the 
tissues destroyed.” He also takes occasion to observe, “ that 
tissues immersed in water are destroyed by even moderate 
pressure; but that in a viscid medium (such as glycerine or 
syrup) the requisite pressure can be borne not only without 
injury or impairment of the discrimination of their parts, 
but with advantage to their detail.” One very great advan- 
tage which results from the use of glycerine for the prepara- 
tion of textures is, that however they may swell in it after 
prolonged immersion, a sufficient soaking in water will 
always restore them to their normal condition. Another is, 
that on account of its very high refractive power, it is 
peculiarly fitted for the preparation of structures to be in- 
vestigated by polarized light, with the same advantage as in 
the preceding case, that they are still amenable to all other 
modes of inquiry. 
The caustic alkalies—potash, soda, and ammonia, are 
solvents of all animal textures except chitine, and perhaps 
bone. Asin nearly all cases a softening action, with little 
or no alteration of tissue, precedes the solvent action, these 
agents, and especially the first two, have their uses. Under 
their influence “a condition is induced very favourable to 
the imbibition of water, which afterwards penetrates very 
rapidly, so that cells swell up and burst.” They may be 
used either with or without heat, and more or less dilute. 
There is one disadvantage attending their use, that objects 
can with difficulty be preserved after soaking in them. 
Heat, applied either by the aid of hot water or steam, or 
the sand-bath, or a bath of fusible metals, or of melted lead, 
is a very efficient means of softening horny substances, whale- 
