36 MANUAL OF BACTERIOLOGY. 



dropped into a 5 per cent, carbolic acid solution and after- 

 ward sterilized by steam. 



Hanging-block preparations, which \vere introduced by 

 Hill, 1 make use of a cube of nutrient agar instead of a drop 

 of fluid. Bacteria are distributed on the surface of the agar, 

 which is then applied to a cover-glass, and mounted like 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 slips of glass. Such slips of glass are 

 generally called 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 so- 

 lution, and afterward washed thoroughly in distilled water, 

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

 be kept in a bottle of alcohol. 



CLEANING FLUID. 



Potassium 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, com- 

 mercial 230 c.c. 



For most purposes it is sufficient to wash the cover-glass 

 in alcohol containing 3 per cent, of hydrochloric acid. It 

 should then be wiped clean with a piece of linen cloth. 

 "Whenever it is taken into the ringers it should be held by the 

 edges, never by the flat surfaces. As far as possible it 

 should be handled with the forceps. It can be used very 

 conveniently in the form of forceps known as the Cornet 

 forceps, or in the modification devised by Stewart. Bac- 



1 Journal of Medical Research, Vol. VII., March, 1902. 



