The tissues should then be removed, the bottle emptied and well washed, and, being refilled with clean spirit, the preparations are again to be replaced. This may occasionally be repeated, until the spirit becomes and remains perfectly bright and clear. The specimens are then ready for section. 6. _Special Methods of Hardening._—The brain (§ 27), spinal cord (§ 43), liver (§ 34), and several other organs, etc., require special methods of hardening, details of which will be found in the paragraph devoted to each. In the case of _injected_ preparations, the best plan is to harden them in alcohol from the outset, beginning with weak spirit, and gradually increasing the strength as the hardening proceeds. When the object has been injected with Prussian blue, a few drops of hydrochloric acid should be added to the alcohol to fix the color. It may here be observed, that specimens of _morbid tissues_ require, as a general rule, a shorter immersion in chromic acid solution than healthy tissues do. A very small degree of overhardening speedily renders them brittle and useless. They should, therefore, be removed from the acid medium at the end of ten days or a fortnight, and their further hardening carried on by means of alcohol. 7. _Cutting Hardened Tissues by Hand._—Our material being now reduced to a fit condition for cutting, let us proceed to consider the several methods by which this may be effected. The readiest and most simple plan, if the piece be large enough, is to hold it in the left hand, and, having brought the surface to a perfect level by cutting off several rather thick slices, endeavor to cut a thin section by the aid of a very sharp razor, the blade of which must be kept well _flooded_ with spirit. As in the use of _Valentin’s_ knife, so here, great care must be taken steadily to _draw_ the blade across the tissue, every effort being made to avoid _pushing_ the knife, else the section will be _torn off_, instead of being _cut_. Though this method[3] is of very great importance for many purposes, yet a considerable degree of manipulative skill is required to enable the operator to obtain anything like perfect sections by its means, and, unfortunately, this skill is acquired by very few persons indeed, even after much practice. If the piece which it is desired to cut be too small to be conveniently held in the hand, it may be imbedded in paraffine in the manner already described (§ 3). A very simple imbedding agent, and one of the greatest practical value, is a strong solution of gum arabic, which, upon being dehydrated either by ordinary drying or the action of alcohol, soon acquires such a degree of hardness as to permit it (with the imbedded tissue) to be cut with ease. As this method of imbedding, however, is most frequently resorted to where, by its means, special difficulties have to be overcome, a full description of the process (§ 35) will be deferred until such special cases come to be spoken of. Footnote 3: Note C.