86 



THE ART OF MAKING MICROSCOPE SLIDES 



Coral 



they must, therefore, be completely dehj'- 

 drated and cleared as though whole- 

 mounts were to be made of them (Chap- 

 ter 6). This dehydration is very slow so 

 that, after about two weeks in 70% alco- 

 hol, they should be placed for a further 

 two weeks in 95% alcohol before being 

 transferred for still another week into ab- 

 solute alcohol. As considerable volumes 

 are required it may be found more eco- 

 nomical to substitute anhydrous acetone 

 for the absolute alcohol. 



The writer prefers to embed in Canada 

 balsam but this must be freed of its con- 

 tained essential oils if it is to become hard 

 enough for grinding. It is very difficult to 

 melt dry Canada balsam without obtain- 

 ing a mass full of air bubbles and it is, 

 therefore, better to take a pound or two 

 of the natural balsam, place it in a wide 

 evajjorating dish and heat it to about 

 120°C., with due precautions against fire, 

 until such time as a small drop placed on a 

 cold plate hardens rapidly to a material 

 which will crack and chip, rather than 

 bend, when a knife is applied to it. 



A solution of about 30% by weight of 

 this essential-oil-free Canada resin in ben- 

 zene is also required but may be made up 

 from the commercial dried product. 



After the specimen has been completely 

 dehydrated it must, of course, be de-alco- 

 holized in some material which is miscible 

 with the resin and it is suggested that 

 benzene be used. Three changes of ben- 

 zene, with about a week in each, will be 

 required ; and it is desirable that some de- 

 hydrating agent, preferably silica gel or 

 calcium sulfate should be kept in the ves- 

 sel to remove the last traces of water. 

 When the specimen is completely impreg- 

 nated with benzene it is transferred to the 

 solution of drj' Canada balsam in benzene 

 and left there for a week or two until it is 

 imi:)regnated. The solution is then trans- 

 ferred to an open vessel and warmed 

 gently until as much as possible of the 

 solvent has been removed. Care must be 

 taken never to raise it to the boiling point 

 of the solvent or bubbles may occur in the 

 actual specimen which will wreck the sub- 

 sequent preparation. The specimen is then 

 immersed in molten balsam, which is 

 maintained at about 100°C. until no fur- 



ther diffusion currents are seen. The 

 beaker is then cooled until the balsam is 

 completely hardened. The only practical 

 method of removing this hardened block 

 of clear balsam containing the specimen 

 is to crack the beaker away from it with a 

 hammer. 



One is now left with a solid block of bal- 

 sam containing the coral, and a saw is used 

 to trim the specimen to shape. This trim- 

 ming should result in a rectangular block, 

 if we are dealing with Astrangia danae, of 

 about a 3'^-inch side, by one inch long, 

 with the polyp protruding from one end. 

 Canada balsam is not easy to saw and it is 

 recommended that the blade be kept lu- 

 bricated at all times with a weak soap 

 solution. 



This rectangular block is now treated 

 exactly as though it were a piece of bone 

 and a series of slices of from 3^^ to one mil- 

 limeter thick cut from it. These slices can- 

 not, however, be handled all at one time in 

 the manner described in the last example, 

 but must be handled individually. 



Each slice is therefore taken and ground 

 flat with carborundum used in the manner 

 described in the last example. Instead, 

 however, of having the section cemented 

 on a slide, it is held on the ball of the first 

 finger and rubbed backward and forward 

 until it is flattened. When it is flat, it will 

 not be satisfactory to wash it under the 

 tap, because many of the carborundum 

 grains will be embedded in the balsam and 

 one should, therefore, take a rag moist- 

 ened with benzene and wipe the surface 

 carefully until the carborundum grains 

 are seen to be removed. Great care should 

 be taken to do this in such a manner that 

 the flatness of the center of the section, 

 where there are only the soft parts em- 

 bedded in balsam, is not disturbed. The 

 section is then rubl^ed up and down with 

 the finger using pumice on wood, and 

 again washed and cleaned. These speci- 

 mens do not polish well on leather and a 

 sheet of velvet (which may be gummed to 

 a wooden block) should be substituted. 

 This velvet is Uberally lubricated with 

 white rouge in water and a polish put on 

 the lower surface of the section. If the 

 white rouge is sufficiently diluted with 

 water, it is unlikely to become embedded 



