It has been before mentioned that some have objected to chloroform and balsam, believing that it became _clouded_ after a certain time. Perhaps this may be accounted for in part by the fact that almost all objects have a certain amount of dampness in them. Others are kept in some preservative liquid until the time of mounting, and these liquids generally contain certain salts (Chapter IV.). If this dampness, as well as all traces of these salts, however small, are not totally removed--the former by drying, the latter by repeated washings--the addition of chloroform will render the balsam much more liable to the cloudiness than when balsam alone was used, as before mentioned.
This mode of employing the balsam, however, will not be always applicable, as _chloroform_ acts upon some substances which balsam _alone_ does not. Some salts are even soluble in it, the crystals disappearing after a few days or weeks, whereas in the balsam alone they are quite permanent. Experience is the only guide in some cases, whilst in others a little forethought will be all that is required.
The particular methods used for certain objects may be now entered upon. Many of the Diatomaceæ and fossil Infusoria, as they are sometimes termed, are mounted dry, and cleaned in the way described in Chapter II. Others are almost always placed in balsam, except where they are intended to be used with the lieberkuhn and dark background, by which means some of them are rendered exquisitely beautiful. The usual way of mounting them in balsam is as follows:--Take a drop of the water containing them, place it upon the slide, and evaporate over the lamp, whilst with a needle they may be dispersed over any space desired. When they are thoroughly dry, drop a little balsam on one side, and exclude the bubbles. The slide may then be warmed to such a degree that the balsam, by lifting the glass at one end, will be carried over the specimens, which may then be covered with thin glass, made warm as before described. Where the objects are quite dry, and loose upon the glass, it requires great care in placing the cover upon them, otherwise they are forced to one edge, or altogether from under it, in the wave of the balsam. For this reason, Professor Williamson adds a few drops of gum-water to the last washing, which causes them to adhere sufficiently to the glass to prevent any such mishap.