EXAMINATION OF BACTERIA WITH THE MICROSCOPE. 25 



a hanging-drop. The bacteria are kept in a layer close to the 

 glass, where growth may be studied. 



Cover-glass Preparations.— The study of bacteria with 

 the microscope is for the most part done by means of smears 

 made upon thin cover-glasses. It is best to obtain the kind 

 sold by dealers as No. i, f-inch squares. 



The cover-glass may be cleaned best by immersion in a 

 mixture of sulphuric acid and bichromate of potassium solu- 

 tion, and afterward washed thoroughly in distilled water, and 

 finally in alcohol. A stock of clean cover-glasses may be kept 



Cleaning Fluid. 



Potassiuln bichromate . 40 grams. 



Water . 150 c.c. 



Dissolve the bichromate of potassium in the water, with 

 heat; allow it to cool; then add slowly and with care 

 sulphuric acid, commercial. . 230 c.c. 



in a bottle of alcohol, or perhaps preferably in alcohol contain- 

 ing 3 per cent, of hydrochloric acid. When they are needed for 

 use they should be wiped clean with a piece of linen cloth. If 

 they are heated at the time the preparations are made the 

 bacteria will be found to spread more readily than on the cold 

 surface and to adhere better. Whenever it is taken into the 

 fingers it should be held by the edges, never by the flat surfaces. 

 In spreading bacteria upon it and in all subsequent manipula- 

 tions, as staining, the cover-glass should be handled with the 

 forceps. It can be used very conveniently in the form of for- 

 ceps known as the Cornet forceps, or in the modification de- 

 vised by Stewart. Bacteria may be placed upon the cover- 

 glass by allowing the glass to fall upon one of the colonies of 

 bacteria, on a gelatin or agar plate (see page 98), which will 

 adhere to it in part, producing an "impression preparation" 

 (German, Klatschpreparat) . Such a preparation, after drying 

 in the air, is to be fixed by passing it through the flame three 



