18 The Microscope. 



to encircle the edge of the cover. This ring of cement not only 

 adds to the appearance of the slide, but it holds the cover in 

 place and prevents the loss of the mounting medium when that 

 medium happens to be a liquid that does not become hard by the 

 evaporation of some of its parts. A cement, therefore, is an 

 important thing to the microscopist that mounts his own objects, 

 and it should possess certain qualities, without which it ma}' be 

 useless and dangerous ; dangerous because, after the micro- 

 scopist lias applied it and relied upon it to keep the mount in 

 good condition, it maj^ crack or chip from glass, or become so 

 brittle by drjdng that a slight shock to the slide may make it 

 and the entire mount leap into the air. This is not a rare oc- 

 currence with certain mixtures sold by the dealers and recom- 

 mended in the books. A good cement should be suftlcieiitly 

 fluid to flow readily from a camel's hair brush; it should dr}' 

 quickly ; it should cling firmly and evenl}- to the glass ; it should 

 not easily crack, so as to allow the air to enter or the liquid 

 mounting, medium to exude, as some media liave the great 

 tendenc}^ to do, and it should have no affinity for the mounting 

 medium that it surrounds, and no chemical action upon it. 



But these are qualities which the beginner will learn about by 

 experience after he has made the mount and laid it aside for 

 a time Yet there is another qualit}^ which will make itself 

 apparent immediately, and it will be one of the greatest annoy- 

 ances to the beginner, as it continues to be to the advanced 

 worker. This is the tendency of the cement to run under the 

 cover and to spoil the preparation. There are exceedingly few 

 cements that will not do this reprehensible thing. The micro- 

 scopist prepares his object, and with a good deal of pleasant 

 anticipation applies the cement around the edge of the cover, 

 and he has the chagrin to see the stufi" slowly but surely begin 

 to creep under and gradually to spread through the whole prep- 

 aration, or at least far enough to ruin the appearance of the 

 mount, if it does not destroy the object itself. A cement that 

 has the greatest tendency to keep where it is placed and shows 

 the least tendency to run under the cover is, other things being 

 equal, the best for mounting purposes. But the cement that 

 will never run under does not exis'. It is for the careful micro. 

 scopist to use so carefully those that he selertts that they shall 

 rarely or never do this obnoxious thing. Yet every mounter of 



