176 Methods of Observing Micro-organisms 



dean. It is therefore best to clean a large quantity in ad- 

 vance of use by immersing them first in a strong mineral 

 acid, then washing them in water, then in alcohol, then in 

 ether, and finally keeping them in ether until they are to 

 be used. Except that it sometimes cracks, bends, or 

 fuses the edge of the glass, a more convenient method is 

 to wipe the glasses as clean as possible with a soft cotton 

 cloth, seize them with fine-pointed forceps, and pass them 

 repeatedly through a small Bunsen flame until it becomes 

 greenish-yellow. The hot glass must then be slowly elevated 

 above the flame, so as to allow it to anneal. This manceuver 

 removes the organic matter by combustion. It is not 

 expedient to use covers twice for bacteriologic work, though 

 if well cleansed by immersion in acid and washing, they 

 may subsequently be employed for ordinary microscopic 

 objects. 



The fragility of the covers and their likelihood to be 

 broken or dropped at the critical moment, make most 

 workers prefer to stain directly upon the slide. The slide 

 should be thoroughly cleaned, and if the material to be 

 examined is spread near one end, the other may serve as a 

 convenient handle. The slide is also to be preferred if 

 a number of examinations are to be made simultaneously 

 or for comparison, as it is large enough to contain a number 

 of ''smears." 



Simple Method of Staining. The material to be ex- 

 amined must be spread in the thinnest possible layer upon 

 the surface of the perfectly clean cover-glass or slide and dried. 

 The most convenient method of spreading is to place a 

 minute drop on the glass with a platinum loop, and then 

 spread it evenly over the glass with the flat wire. Should 

 it be stained at once it would all wash off, so it must next 

 be fixed to the glass by being passed three times through a 

 flame, experience having shown that when drawn through 

 the flame three times the desired effect is usually accom- 

 plished. The Germans recommend that a Bunsen burner 

 or a large alcohol lamp be used, that the arm describe a 

 circle a foot in diameter, each revolution occupying a second 

 of time, and the glass being made to pass through the flame 

 from apex to base three times. This is supposed to be exactly 

 the requisite amount of heating. The rule is a good one for 

 the inexperienced. 



Inequality in the size of various flames may make it de- 



