Another very frequent cause is the transference of air bubbles with the mounting medium on the glass rod. This occurs especially if the rod be fused to the stopper. The proper bottles to use, both for Farrant’s medium and balsam are “balsam bottles” which have no stopper, but the mouth is closed by a glass cap which fits accurately (fig. 9). A short glass rod is attached to the cap, and is used to transfer the medium to the slide. [Illustration: Fig. 9.--Balsam bottle.] *Treatment of folded sections.*--The folding may be due:-- (1.) To the section having creased through being cut with a knife whose surface was not perfectly smooth. This is best remedied by placing the section in methylated spirit for a minute, and then transferring it to a bowl of clean water, when the section will rapidly rise to the top, and spread itself out flat on the surface of the water, in consequence of the alcohol rapidly diffusing out at the edges into the surrounding water. (2) To the section containing a large amount of *fat*, as in those of the skin and subcutaneous tissue. The fat may be removed from the fat cells without materially altering the appearance of the section. This is done by dehydrating the section in alcohol, and then transferring to a watch glass containing ether or chloroform to extract the fat. The tissue should be washed free from ether in the alcohol and then transferred to the bowl of water, and allowed to float out. This process does not interfere with subsequent staining operations. MOUNTING MEDIA. *Farrant’s solution*:-- Gum Arabic (picked, colourless) } Glycerine } equal parts. Water } In making this solution the best gum arabic must be used, and only the clearest pieces of this. “Powdered gum acacia” should be avoided, as though it looks white it often yields a brown mucilage, and besides is frequently adulterated with starch, &c. The glycerine and water should be mixed and the gum arabic added. The mixture should be allowed to stand for some weeks, with frequent stirring until the whole of the gum is dissolved. Then allow it to stand for a week or two longer in order that the dirt may subside, and the bubbles rise to the top. The scum should be removed and the clear fluid decanted from the sediment into a “Balsam bottle” (p. 58) containing a few drops of a saturated solution of arseniate of sodium and a small lump of camphor. If properly made it is an extremely useful mounting reagent. It does not clarify the tissues too much, and in consequence of its containing gum it dries at the edges and cements the cover-glass more or less firmly in a week or two. If this is not the case the medium contains too much glycerine and more gum must be added to compensate for this. This drying at the edge prevents any further evaporation while the glycerine keeps the section permanently moist.