the whole subject of physiological microscopy, no treatise with which we are acquainted is at once so plain, practical, and profound. 18. _Employment of Freezing Microtome._—A very suitable object with which to demonstrate the method of using this form of microtome will be afforded us by a portion of intestine, say of the ileum of a cat or dog. Suppose we have some of this in methylated spirit—let us select a piece of about half an inch in length. Our first care will be to deprive this of its spirit; for so long as the tissue remains impregnated with alcohol it would, of course, be impossible to freeze it. We will, therefore, throw it into a large basinful of water, and leave it there for twenty-four hours, during which time it would be as well to change the water once or twice. We shall now require a strong solution of gum. This, which should have been made some time previously, may be prepared by placing a quantity, say three or four ounces, of ordinary gum arabic in a glass beaker, and adding sufficient water to cover it—the mixture must be stirred occasionally with a glass rod until solution has taken place, which will be in a few days. If _necessary_ a little more water may be added, but so long as the gum will pour from vessel to vessel, it cannot well be made too strong. Mucilage, by keeping, is very apt to become sour and mouldy—this may be prevented by adding to each ounce of the water with which it is prepared about half a grain of salicylic acid. We now pour some of this mucilage into a small vessel—an egg cup will answer very well—and into it transfer the piece of ileum from the water. Here we must allow it to remain for a time sufficient to permit of its becoming thoroughly saturated with the gum, for which purpose some hours will be necessary. When this soaking has been accomplished we will prepare the microtome, which we will assume to be Rutherford’s. In the first place it will be necessary to remove the plug—which is to be done by turning the handle connected with the screw until the plug rises so high in its tube that it may be grasped with the fingers and removed, when it is to be well smeared all over with sperm oil and replaced. This is done to prevent any unpleasant adhesions taking place whilst the freezing is going on. We must next depress the plug, so as to convert the upper part of the tube into a kind of “well” of sufficient depth to hold our specimen. It will now be very advisable to look carefully into this _well_ and observe whether the plug fits accurately into the tube (§ 8) for if there be any interval between the two it will give rise to much subsequent annoyance, as the gum penetrating this interstice will there become firmly frozen into irregular patches, which will so interfere with the even gliding of the plug within its tube as to cause the former to ascend in such an irregular and jerky manner as to be utterly destructive of all accuracy in the cutting. If this defect be