42 EXPERIMENTAL DAIRY BACTERIOLOGY 



layers of cotton cloth. The glass covers are then placed on a 

 flat piece of Russia iron and heated over a Bunsen flame for 

 one hour. The iron should not become red-hot. The prolonged 

 heating destroys all traces of fat. The clean covers should be 

 handled with forceps, for if touched with the fingers a film 

 of grease is left. If a drop of water is placed on a perfectly 

 clean cover glass, it will spread in a film over the entire sur- 

 face. Store the cover glasses in a clean Petri dish. 



Slides and covers that have been used may be cleaned by 

 placing them in turpentine to remove the oil, then in a clean- 

 ing fluid composed of 20 grams of potassium bichromate, 

 100 cc. of water, 100 cc. of sulphuric acid. They should be 

 digested in this mixture until the organic matter is destroyed. 

 Rinse until free from the cleaning mixture, and then treat as 

 new. The slides and covers may also be cleaned by placing 

 in alcohol and adding 23 volumes of concentrated nitric 

 acid, allowing them to stand in this for some time, rinsing, 

 rubbing to remove any organic matter not destroyed, and 

 wiping from alcohol. The addition of the acid should be 

 made out of doors or in a hood, on account of the fumes 

 given off. 



Exercise. Clean | ounce of cover glasses and 50 slides. 



Staining solutions. The size and transparency of the bac- 

 teria make their differential study difficult in unstained prep- 

 arations. In order that their morphology may be more readily 

 determined, the use of stains or dyes is resorted to. The stains 

 usually employed are the basic aniline dyes. For ordinary 

 purposes methylene blue, fuchsin, gentian violet, and bismarck 

 brown are sufficient. 



The stock solutions kept in the laboratory are made by 

 preparing a saturated solution of the various dyes in 95 per 



