ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 715 



filtering the viscous mass, for, notwithstanding tho caution always given 

 against stirring the mass to mix it thoroughly, in my own exi^crience the 

 bubbles formed in stirring have uniformly disajipeared on long standing 

 ill a warm room. Air-bubbles in the comj)leted mount, however, exhibit 

 all the obstinacy with which they have been credited when the mass is 

 prepared on the formula commonly given, viz. two parts each by weight 

 of gum acacia and distilled water, and one part of glycerin. 'I'he gum 

 is dissolved in the water, the glycerin added, the mass filtered, and a little 

 camphor added to prevent mould. This makes a quite viscous mass which 

 quickly dries around the edge of the cover, but from which air-bubbles 

 cannot be driven out nor poked out if once imprisoned under the cover. 



For such objects as are usually mounted in pure glycerin a much 

 thinner preparation of Farrant's medium is very convenient, and is made 

 by simply increasing the proportion of glycerin to gum. Another useful 

 medium, which dries readily but shrinks more than the otheis, is made 

 by taking by weight 6 parts gum, 4 parts white sugar, 16 parts water, 

 and 6 parts glycerin, prepared as described. A still further modification 

 is made by taking 8 parts gum, 4 parts white sugar, 2 parts gelatin, 

 20 parts water, and 12 parts glycerin. Dissolve the gelatin first, then 

 add the gum and sugar, and lastly the glycerin. This mass never dries 

 completely hard, but only to a tough, leathery consistence. In all cases 

 a little gum camphor, phenol, clove-oil, or thymol should be added to 

 the completed mass to prevent fungoid growth. 



In the preparation of Farrant's medium on any formula, much time 

 and annoyance may be saved by making the watery solution of gum, &c., 

 much thinner than it is required to be, and after filtration evaporating it 

 to the consistence desired, and then adding the glycerin. I always add 

 to the water in the -beginning an ounce or so of a weak solution of chloral 

 hydrate, and add gum thymol to the finished mass ; a piece the size of a 

 large pin-head will do for an ounce of medium. 



Jn mounting in any of these gum media, much trouble is saved by 

 first macerating the object in some of the thin medium for a longer or 

 shorter time according to its nature — longer for dense objects than for 

 thin ones — and then arranging the object on the slip in some of the thin 

 medium, allowing most of the water to evaporate (protected from dust), 

 and then adding the thick medium and applying the cover, using a light 

 spring clip to retain it in place. Air-bubbles will not be included by 

 this method. 



If a surplus of the medium was used so that much has escaped around 

 the cover, this excess should be cleaned away within twenty-four hours 

 after the cover was placed, while it is still soft and tough. If the clean- 

 ing is delayed until the mass outside the cover is hard, the cover will 

 often be moved or piilled out of position by the removal of the outer 

 mass. As soon as the jjartially cleaned slide has become quite dry, the 

 slip should be placed on a turntable, and the slide cleaned close up to 

 the cover, using a knife-blade or chisel-point to cut away the gum, and a 

 moist rag or folded blotter to finish. Then add successive finishing rings 

 of some resinous cement. Objects thus mounted will prove as durable as 

 balsam mounts; there will be no shrinkage or distortion of soft jmrts, 

 as often occurs with objects in glycerin ; the most delicate and colourless 

 of structural details are well shown, and the objects photograph extremely 

 well. 



Air-bubbles need not be included in the mounts, but if unfortunately 



