point has been exceeded, the tissue will have become so brittle as to crumble before the knife. For the purpose of hardening animal tissues, the student has at his command two principal agents, namely, alcohol and chromic acid, each of which possesses advantages of its own, but the use of each of which is also attended by its own inconveniences. Thus, by the use of alcohol, there is very much less risk of overhardening the specimen than if chromic acid had been employed. Alcohol, however, though a capital indurating agent in some instances, does not answer so well in many others. Chromic acid is, therefore, to be preferred for general use. It is, however, a very delicate agent to manage, for unless the greatest care be taken it is exceedingly likely to overharden tissues submitted to its action, and when this happens the specimen becomes utterly useless for cutting, as there is no known means of removing the extreme brittleness which it has acquired. By taking the precautions now to be given, this overhardening may generally be avoided. Let us harden a portion of some viscus, say the kidney, for instance. Suppose we cut from the organ five or six small pieces (from half to three-quarters of an inch square, _not larger_). These must be placed in a mixture of equal parts of methylated spirit and water for three days, at the end of which period they may be transferred to a solution of chromic acid, made by dissolving twenty grains of the pure acid in sixteen ounces of distilled water. The solution should be kept in a wide-necked bottle furnished with a glass stopper. At the expiration of seven days, pour off the solution and replace it by fresh. At the end of another week, carefully examine the immersed tissues, and by means of a sharp razor see if they have acquired the necessary degree of hardness to allow of a section of _moderate_ thinness being made. If so, remove the pieces and put them into a stoppered bottle containing from six to eight ounces of methylated spirit. If, however, the hardening be found not to be sufficiently advanced, the chromic acid solution is to be poured off and again replaced by fresh. It will now be necessary to examine the tissues at intervals of about two days, until they are found to be sufficiently hard, when they must be transferred to the spirit. Under no circumstances, however, should they be permitted to remain in the chromic acid longer than the end of the third week, and though they should at this time appear not to have undergone sufficient induration, yet it will be advisable to transfer them to the methylated spirit, which in a short time will _safely_ complete the process of hardening, without any risk being run of the tissue becoming ruinously brittle. It will be noticed, that when the specimens have been transferred to spirit, the latter will in a day or two become of a deep yellow color, whilst a thick flocculent deposit falls to the bottom of the bottle.