2l6 HOW TO WORK 



deliquesce on exposure to air, must be dried in vacuo, removed 

 quickly to a cell, the cover of which must be firmly cemented down 

 at once. Some crystals may, however, be dried and mounted in 

 Canada balsam ; others, such as oxalate of lime, cystine, triple 

 phosphate, &c., can be well preserved in aqueous solutions, contain- 

 ing a little acid in the case of the two former substances, or an 

 ammoniacal salt, in the latter instance, in which the crystals are 

 known to be insoluble. Crystals which contain water of crystallisa- 

 tion must be preserved in a drop of the mother-liquor ; but in many 

 instances they alter much in form, and when we come to examine 

 them, instead of finding a great number of small, well-formed crystals, 

 as when the preparation was first put up, nothing remains but one or 

 two large ill-shaped ones. The concentrated mother-liquor often acts 

 upon the cement with which the glass cover is fixed on the cell, and veiy 

 soon air enters, and the preparation is destroyed. Many crystals may 

 be preserved in strong glycerine without much change taking place. 

 I have some crystals of Guinea-pig's blood which have been preserved 

 for many years in this medium and have undergone little change. 



Of the Hardening Properties of Chemical Solutions. 



317. Of the Hardening Properties of Different Chemieal Solu- 

 tions. The consistence of many tissues is so soft that it is absolutely 

 impossible to obtain a thin section; while, by tearing off a small 

 piece, the relations of the component parts is usually so much altered, 

 as to render the specimen useless for the purpose of examination. In 

 this case our only chance is to harden the texture by some reagent in 

 such a manner that, although its microscopical characters are not 

 altered, a thin section may be readily obtained. 



The solution employed for hardening a tissue will depend upon 

 the character of the texture itself. Many tissues may be immersed 

 in alcohol, others may with advantage be soaked in a weak solution 

 of chromic acid. Various saline solutions are also sometimes em- 

 ployed, but in consequence of the alteration they produce in the 

 texture of the substance, they are not well adapted for many micro- 

 scopical specimens. Boiling in water, is often resorted to for the 

 same purpose. In this way very thin sections of such textures as 

 muscular fibre may be obtained. These may afterwards be rendered 

 transparent by being soaked in syrup or glycerine, or by the addition 

 of a little solution of caustic soda or potash ; nitric acid and a solu- 

 tion of perchloride of iron have also been employed for hardening 

 some tissues. 



The hardening properties of the solutions just referred to, depend 

 essentially upon their power of coagulating albuminous substances? 



