9. In clamping the instrument to a table, or other support, care should be taken that the zinc plate is in a horizontal position. If the plate be not horizontal, the gum will tend to run to one side. The arrangement made for cutting and embedding sections consists of a cylindrical tube (Fig. 235_a_) fitting into the principal well of the microtome, within which is a hinged plate, upon which the screw acts, as in an ordinary vice. To bring this into use the freezing apparatus must be first removed, and the embedding tube placed in the well, and firmly pressed into place. Staining Animal Structures. Specific stains are chiefly employed to assist the eye in distinguishing one elementary tissue from another. It is therefore necessary to stain all structures, as certain parts are seen to have a special affinity for one colouring agent rather than another, whereby they become more deeply stained, and consequently more clearly differentiated. For staining animal structures, borax, carmine, and hæmatoxylin are more frequently employed than others. The formulæ for each will be found in the Appendix “Formulæ and Methods.” _Staining Process._--Place the section in distilled water to wash away the alcohol; place a little of the carmine in a watch glass, and immerse the section in it for four or five minutes; then remove it to a solution composed of methylated spirit five parts, hydrochloric acid one part; shake well together. This solution should be kept ready for use. Immerse the section in this solution and leave it to soak for about five or ten minutes if over-stained, until the desired tint has been obtained. Sections of skin and fibrous tissue may be left until nearly all colour is removed, the glands and hair follicles will then be brought out more clearly. The section must be transferred to methylated spirit to remove all traces of acid, then to oil of cloves contained in a watch glass, lift the section from the methylated spirit by one of the _lifters_ (Fig. 250), and carefully float it on the oil, in which it should be allowed to remain for about five minutes. This is the clearing process, the object of which is to remove the spirit and prepare the section for mounting in Canada balsam. First, however, place the section in filtered turpentine to wash away the oil of cloves; this is found to answer better than another plan adopted, that of removing the section from the oil of cloves and mounting it in balsam direct. The oil, however, has a tendency to darken the balsam. _Logwood or Hæmatoxylin Stains_ (see Appendix for the several formulæ). Staining by this agent is effected as follows:--