ZOOLOaY AND BOTANY, MICROSCOPY, ETC. 



161 



the flame of an ordinary Bunsen burner. The material is next placed 

 in a porcelain crucible, varying in size according to the amount of 

 substance required. About twice the volume of the solution of gum 

 copal and ether should then be poured upon it, always taking care to 

 press the stopper of the bottle well in afterwards. 



The crucible is next placed upon the hot plate, care being taken 

 to have a moderate heat at first, and to allow the mass to simmer 

 till the ether has partly evaporated, when a greater heat may be 

 applied. If the substance is a fine sand or ooze, it must be well 

 stirred with a needle-point or small knife, otherwise it will stick to 

 the bottom of the crucible, and not allow the gum copal to mix with 

 it. If it is a soft, porous, or decomposed rock, it will only be 

 necessary to turn it a few times, so that the solution may thoroughly 

 penetrate all the pores. Great care must be taken during this part 

 of the operation, as the cement is very inflammable, and therefore 

 caution is essential, not only in stirring, in consequence of the gum 

 having a tendency to stick to the sides of the crucible, but also in 

 removing the stirring-needle to avoid contact with the flame. 



After nearly all the ether has evaporated, the substance, if it is 

 of a granular nature, should form a thin, stringy mass when stirred, 

 and the operator can judge whether sufficient of the gum remains to 

 cement the grains together ; if too much has been applied, more of 

 the substance and a small quantity of pure ether must be added, and 

 the whole boiled over afresh. When there is a sufficiency of gum, 

 the mixture should be kept boiling and well stirred till it becomes of 

 a reddish or brown colour ; sometimes it is difficult to discern the 

 colour, as the substance interferes with it, but it can be seen in most 

 cases. The operator, however, can easily ascertain whether it has 

 been sufficiently boiled and has attained the necessary consistency, 

 by taking a little out on the point of a knife, and rapidly cooling it 

 by pressing it against some cold surface, or holding it a short time 

 in water. If it hardens immediately, it has been boiled enough. 



The crucible can now be taken off, and while yet warm, tho 

 substance should be scraped out with a knife, and rolled or pressed 

 with the fingers into an oblong mass ; it is then ready for moulding, 

 or it can be laid aside and 



moulded at any time by Fig. 38. 



gradually softening on a a h 



piece of glass or in a 

 porcelain dish upon the 

 hot plate. The moulds 

 are easily made by cutting 

 strips of ordinary tin 4 in. 

 by 3/4 in., bent tightly 

 over a round iron rod, a closed opem 



small slit being cut on 



one side of the tin to allow the wire connected with the mould to sink 

 below the surface of the rim ; this permits the mould to stand level and 

 close to the glass plate ; take the tin off the iron rod and bind it firmly 

 round with fine copper wire : it is then ready for use (fig. 38 a). 



Ser. 2.— Vol. VI. M 



