124 ELEMENTARY CHEMICAL MICROSCOPY 



etched, corroded and scratched and should then be discarded. 

 Before being used, new slides should be dropped in warm chromic 

 acid cleaning mixture, washed free from all acid in hot distilled 

 water, drained, dried, in a locality free from dust, and when dry 

 stored in covered boxes or wide-mouthed bottles. The simplest 

 test which can be applied to an object slide to determine its fit- 

 ness for use is to place upon its surface a small drop of distilled 

 water and slowly tip the slide; if the drop flows readily across, 

 leaving an unbroken streak of water, the surface is clean; if, 

 however, the drop refuses to flow or if upon flowing it immedi- 

 ately breaks away from the mother drop, the surface of the slide 

 is dirty or greasy and is not fit for microchemical manipulations. 

 Passing a greasy object slide slowly through the flame of a Bun- 

 sen burner will often render it fit for use. 



All cloths used in wiping slides, etc., must be free from lint 

 and washed absolutely free from starch, dextrine or other fillers 

 which may have been present. The so-called " glass- toweling " 

 of commerce, after thorough washing, will be found to be one of 

 the best materials for use. It must be remembered, however, 

 that after handling, any such material takes up sufficient greasy 

 material from the hands as to render it unfit for use; for this 

 reason it will be found convenient to have the toweling cut in 

 short lengths and to mark one side in some manner and always 

 apply the hands to the marked side only. Frequent laundering 

 is essential. 



The difficulty of preparing absolutely clean slides is never 

 fully appreciated until one has tried working with dark-ground 

 illuminators and various types of the ultramicroscope. In the 

 more refined methods of ultramicroscopic investigations it is 

 found that glass slides cannot be made sufficiently free from 

 objectionable surface films for use, and recourse must be had to 

 quartz slides or disks which, after cleaning as described above, 

 are heated to bright redness just prior to being employed. 



Quartz (fused silica) slides may now be obtained from any 

 firm dealing in this material, sufficiently free from air bubbles, 

 to permit using even high powers, and of such transparency as 

 to leave little to be desired. Small slips or tiny cells of silica 



