MICROSCOPICAL TECHNIQUE. 429 



cover inverted by means of a pair of forceps, and lowered on to 

 the middle of a clean slide that has previously been gently warmed 

 in a spirit-lamp flame. If the cover be lowered carefully and 

 gradually, any air-bubbles that may have formed will escape easily, 

 and the cover may then be pressed steadily until the section lies 

 quite flat and uniform. As soon as the slide is quite cool, any 

 exuded balsam may be washed off with a soft rag or camel-hair 

 brush moistened with methylated spirit, and, after a final polish 

 with a dry cloth, a ring of cement may be applied to finish. Thus, 

 though some time is required for the balsam to harden in the pre- 

 liminary stage of this process, that is much more than compensated 

 for by the rapidity with which the slides can be finished off. In 

 the ordinary, so-called quick method, the slides must be left for a 

 day or two, or even longer, before it is safe to attempt to clean and 

 ring them, and there is always a risk of moving the cover or 

 admitting air-bubbles at the last moment. 



Clearing Sections.— The object of this process, as is well-known, 

 is to remove all traces of alcohol from dehydrated sections. Unless 

 this be completely effected, the remaining alcohol will cause a 

 certain amount of cloudiness where it comes in contact with the 

 Canada balsam subsequently used as the mounting medium. The 

 essential oils of bergamot, cedar, and cloves, are generally used for 

 the purpose of clearing, the last-named most commonly perhaps. 

 Bousfield, however, in his Photo-Micrography^ notes that he has 

 long ceased to use essential oils after dehydration. He points out 

 that however high the refractive index of the oil used may be, the 

 ultimate index must be that of the medium used for mounting. In 

 his own particular case this is xylol-balsam, and he prefers to 

 remove the last traces of absolute alcohol from the sections by 

 immersing them in two successive lots of mineral naphtha. This 

 he finds to answer every purpose, and the fact that the fluid is 

 entirely without action upon aniline dyes, with which staining is 

 now so often effected, gives it a special advantage over most 

 essential oils, particularly that of cloves. 



Crystals of Gold. — To procure gold-crystals, as objects for the 

 microscope, proceed as follows : — Make a lo per cent, aqueous 

 solution of neutral auric chloride (Au. CI 3), and of this put a 



