Recent Microscopic Literature. 331 



solvent for cleaning slides. Another plan is to use chloroform, 

 as Mr. Davies thus describes : — " The balsam is exposed to 

 heat until on cooliug it assumes a glassy appearance ; it is then 

 dissolved in pure chloroform until it becomes of the consistence 

 of thick varnish. This liquid is very convenient in some 

 cases, as air bubbles are much more easily got rid of than when 

 undiluted Canada balsam is used. It also dries readily." 



We shall make one more extract from Mr. Davies, relating 

 to the treatment of the Equisetaceas, which are now growing 

 in easily accessible places. He is speaking of their preparation 

 for the polariscope, and tells us, " Some of these plants, in- 

 cluding many of the grasses and Equisetaceae (i.e., horsetails), 

 contain so large a quantity of silica, that when the vegetable 

 and other perishable parts are removed, a skeleton of wonderful 

 perfection remains. This skeleton must be mounted in balsam, 

 the method of preparing which will now be considered." 



u Sometimes the outside of the Equisetum is removed from 

 the plant, others dry the stem under pressure, whilst the 

 grasses of course require no preparation. The vegetable 

 should be immersed in strong nitric acid, and boiled for a 

 short time ; an effervescence will go on as the alkalies are being 

 removed, and when this has ceased more acid should be added. 

 At this point the modes of treatment differ ; some remove the 

 object from the acid, and wash, and having dried, burn it upon 

 thin glass until all appears white, when it must be carefully 

 mounted in balsam. I think, however, it is better to leave it 

 in strong acid until all the substances, except the required 

 portion, is removed ; but this will take a length of time, vary- 

 ing according to the mass, etc., of the plant. Of course, 

 when this latter method is used, the skeleton must be washed 

 from the acid, etc., before being mounted in balsam." 



. M. Chevallier's book is an effort., on a smaller scale and 

 less comprehensive plan, to do for French students what Dr. 

 Carpenter's work on the microscope has accomplished for our 

 own. From his pages we conclude that many desirable ac- 

 cessories for the illumination of both opaque and trans- 

 parent objects that are commonly employed in England are 

 little used in France, and the chapter on Test Objects, mainly 

 founded on experiments so old as those of Dr. Goring, would 

 not in this country be considered up to date. The student is 

 taught, for example, to be satisfied with a definition of the 

 Podura scale, far below what would be given by a fair second- 

 rate quarter-inch of English make. 



M. Chevallier recommends distilling a liquid from Canada 



balsam, in order to obtain a fluid well adapted to thin other 



specimens of the balsam, or to soak objects in, that are in- 



' tended to be mounted in it. He likewise recommends a 



