150 ELEMENTARY CHEMICAL MICROSCOPY 



Great care, therefore, is necessary when very minute amounts 

 of material are to be tested, to avoid being led into serious error 

 arising from the extraction of constituents from the glass slides. 



Object slides of greenish glass, the usual material supplied 

 some years ago, and sometimes still found on sale, are much 

 better, being harder and more resistant to the action of chemicals. 



Standard slides, 3 inches by i inch, are too long and should 

 be cut in half, or half-size slides purchased, since microchemical 

 reactions are generally performed at the corners of the slides, 

 seldom if ever at the center. A full-sized slide cannot be satis- 

 factorily rotated on the stage of the polarizing microscope with 

 the material situated at one corner, since the slide extends too 

 far beyond the rim of the stage; nor can material be heated at the 

 center of the slide without incurring the danger of breaking. 



Object slides of ordinary non-resistant glass rapidly become 

 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 



