92 PATHOGENIC BACTERIA. t> glasses can never be too clean. It is best to immerse them first in a strong mineral acid, then to wash them in water, then in alcohol, then in ether, and keep them in ether until they are to be used. Except that it some- times cracks, bends, or fuses the edges of the glasses, a better and more convenient method of cleaning them is to wipe them as clean as possible, seize them in fine-pointed forceps, pass them repeatedly through a small Bunsen flame until it becomes greenish yellow, then slowly ele- vate the glasses above the flame, so as to allow them to anneal. This maneuvre removes the organic matter by combustion. It is not expedient to use covers twice for bacteriological work, though if well cleaned they may subsequently be employed for ordinary microscopic ob- jects. **~S To return: After the material spread upon the cover has dried, it must be fixed to the glass by immersion for twenty-four hours in equal parts of absolute alcohol and ether, or, as is much easier and more rapid, be passed three times through a flame. Three is not a magic number, but experience has shown that when drawn through the flame three times the desired effect seems best accomplished. The Germans recommend that a Bunsen burner or a large alcohol lamp be used, that the arm holding the forceps containing the cover-glass in- scribe a circle a foot in diameter, and that, as each revo- lution occupies a second of time, the glass be 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. After fixing, the material is ready for the stain. Every laboratory should be provided with several stock-solutions of the more ordinary dyes. These stock-solutions are saturated alcoholic solutions made by adding- 25 grams of the dye to 100 c.cm. of alcohol. Of these it is well to have fuchsin, gentian violet, and methylene blue always made up. The stock-solutions will not stain, but are the standards for the manufacture of the working stains.