The camphor and arseniate of sodium prevent the formation of fungi. Sections preserve their original appearance in this medium for many years. After a long time they are apt to become a little cloudy and granular. Unstained sections should always be mounted in Farrant’s medium, as the Canada balsam process renders them quite transparent. It is suitable for almost any tissue stained or unstained, but sections of the nervous centres require to be mounted in Canada balsam, owing to the opacity of myelin when mounted in glycerine. *Canada balsam solution*:--The medium is made thus:-- The ordinary Canada balsam which is of a treacly consistence is heated gently in a water bath for some hours, to drive off turpentine and other volatile oils. It is then allowed to cool to a yellow vitreous mass. Take of Dried Canada balsam } equal parts. Xylol } Leave till dissolved, stirring occasionally. Unless the solution be perfectly clear, it must be filtered through a very thin paper, previously wetted with xylol. If the medium be too thick more xylol should be added, if too thin, the xylol should be allowed to evaporate until the medium is of the consistence of a thin syrup. If the medium is made too thin much annoyance will be caused by its evaporating at the edge of the cover-glass, leaving an air-space, which will increase daily until the section is left quite dry. This should be remedied by putting another drop of balsam at the edge of the coverslip and allowing it to run in and displace the air. A ring of cement should be put on as early as possible afterwards. The bottle in which the balsam is preserved must be very carefully dried before being filled and then rinsed out with absolute alcohol, and afterwards with xylol. Turpentine or benzol are often used instead of xylol in the preparation of the medium, and in the same proportion, but the latter is less apt to dissolve out the aniline colours from the sections. To mount sections in Canada balsam they must be transferred first to a watch glass containing absolute alcohol or an alcoholic solution of some staining reagent, _e.g._, eosine (p. 72) and left in it, no attempt being made to spread it out, until it is perfectly dehydrated, _i.e._, in about two minutes. It should then be transferred to the clarifying oil on a mounted needle, or on a section lifter, which must be perfectly dry as any spot of moisture that gets on to the section will resist the clarifying action of the oil, and will cause unsightly opaque areas when the section is mounted. Even breathing on the section on its way to the clarifying agent will prevent uniformity of clearing. Should white spots appear in the section while in the oil it must be taken out with as little oil as possible, and again dehydrated in absolute alcohol.