1S93.] NEW-YORK MICROSCOPICAL SOCIETY. 29 



fere with observation, by the coalescence of several minute fixed 

 bubbles into a large movable one. This may lead to harm by 

 weakening the mount, disturbing the object as the bubble rolls 

 about, and tending to foster germ-life for its destruction. 



In cases, however, of extreme care toward their avoidance, 

 the experienced microscopist has been repeatedly surprised and 

 disgusted by the mysterious appearance of minute air-bubbles, in a 

 hermetically sealed mount, from some unknown source. To that 

 source I would call attention, in the film of condensed air 

 and moisture which has been shown to be firmly attached, under 

 ordinary conditions, to the surface of all solid bodies, and which 

 has been best studied on the surface of metal and glass. 



The following precautions are taken in our laboratory against 

 this invisible enemy : 



1. As the air-film can be removed by friction, all plain slides 

 and the interior of cells are briskly rubbed just before using. The 

 microscopist unfamiliar with the difficulty would content himself 

 by merely dusting an apparently clean slide. 



2. The stock of glass covers is thoroughly cleaned at one 

 time by immersion in Seller's solution (one part saturated solution 

 of potassium dichromate in three parts of concentrated sulphuric 

 acid) for about an hour, thorough washing successively in common 

 and then in distilled water, and immersion in strong alcohol. 

 In the latter the covers are allowed to remain, in a wide mouthed 

 glass-stoppered bottle of about thirty cubic centimetres capacity. 

 Just before use each cover is taken out and well rubbed, dried, 

 and placed on a warm mounting-table, so that it may be applied 

 to the mount chemically and microscopically clean and entirely 

 devoid of the air-film, which ordinarily soon becomes condensed 

 upon a cold glass cover. 



3. All preservatives are kept slightly warmed just before use, 

 and the object is soaked in distilled water recently boiled and 

 cooled, and therefore strongly absorbent of air. 



The insertion of a mount in a vacuum, under the bell-jar of a 

 convenient air-pump, for a short time just before it is to be cov- 

 ered, isauseful precaution, especially with an object consisting of 

 more or less tangled fibres, or of a cellular character with partially 

 empty cavities. But I have not found that the condensed air- 

 film can be removed in that way. 



