_c._ _Double-bladed Knife._ This consists of two parallel blades arranged so that the space between them can be changed by means of screws. For firm material the blades should be close together; for soft material farther apart. The blades should be dipped into physiologic salt-solution to fill up the space between the blades. The instrument is then drawn through the tissue, and the section between the blades floated out in salt-solution. Some workers use this double-bladed knife to make a perpendicular cut into the tissue, then turning it to either side to cut the lower edge of the section and removing the blades with the section between them. Both the single- and the double-bladed razors require practice for successful section-cutting. Further, it must be borne in mind that sections of fresh tissue obtained by these methods are unsuited for complicated staining methods and can be used only for the simplest staining processes. Nevertheless, in a fairly large proportion of pathologic conditions it is possible to secure a diagnosis by these methods. _d._ _Freezing Microtome._ Much more satisfactory sections can be obtained by freezing the fresh tissue and cutting it upon the =freezing microtome=. Various types of these instruments are in the market. The freezing is accomplished by the use of _ether_, _ethyl-chloride_ or _fluid carbonic acid gas_. The ether and ethyl-chloride freezing-microtomes consist essentially of a metal plate or hollow box on which the tissue rests and against the under-surface of which a spray of ether or ethyl-chloride is forced by means of a rubber bulb connected with a supply of the freezing agent in a bottle. Such freezing attachments can be attached to any microtome, but special instruments as Jung’s “student’s freezing microtome,” Cathcart’s, Bausch and Lomb’s or Becker’s ether-freezing microtome can be recommended for this purpose. (See Fig. 52.) The tissue to be frozen must be of small size and not more than 3-4 mm. thick. It is placed upon the freezing plate in a drop of water, white of egg or thick gum-arabic, and pressed firmly against the plate. The spray must not be too constant or strong, but should be given with regular pauses of about a second to allow the ether to evaporate. The tissue must be firmly frozen, but not so hard that it crumbles. It is usually difficult to freeze the upper part of the tissue hard enough to give good sections, and this half-frozen tissue must be trimmed off with the knife until good sections are secured. As the sections are cut they are removed from the knife-blade with the finger and put into physiologic salt-solution that has been recently boiled to drive off the air, so that, in thawing, there may be no formation of air-bubbles in the tissue to cause artefacts. If put into strong alcohol diffusion-currents may damage the tissue; a succession of graded solutions should, therefore, be used if the sections are to be fixed and hardened.