54 THE BACTERIOLOGICAL EXAMINATION OF MILK 



in a 10 per cent solution of HCl, and the slides should be 

 thoroughly washed in running water. They should then be placed 

 in a beaker containing a mixture of concentrated sulphuric acid 30 

 parts, bichromate of potassium 30 parts, and water 500 parts, and 

 allowed to remain for some hours, care being taken that they do 

 not cling together by capilliary attraction and so prevent the acid 

 coming in contact with the whole surface of the slide. On removal 

 from the acid they should be thoroughly washed in running water, 

 dried with a soft cloth free from any trace of grease and stored in 

 alcohol until required for use. 



New slides should be carefully dusted, placed in the acid 

 solution, and allowed to soak as above. They should then be 

 thoroughly washed in a stream of running water, dried with a 

 soft cloth free from any trace of grease, and placed in a jar of 

 alcohol until required for use. Care must be taken that from 

 the moment they leave the acid bath they are not handled with 

 the bare fingers except by the edges, and if they are to be sub- 

 sequently used for flagella staining, even this should be carefully 

 avoided. 



It will be found in practice a good plan, when taking new 

 slides into use to mark, at the extreme end of each slide, one 

 surface with a writing diamond or sharp flint, and to make a rule 

 of always using the marked side for the preparation. Unless some 

 such method is adopted it is at times difficult, especially when 

 staining by Gram's method or when the film is very thin, to 

 ascertain on which side the preparation has been made. 



Cover glasses. — If cover glasses are to be used for making 

 preparations upon they should be treated in similar fashion to 

 slides as described above, but if, as recommended, staining is to be 

 carried out on the slide, and the cover glass used only as a cover, 

 it will be found sufficient to clean them between the thumb and 

 index finger with a clean soft rag moistened with a mixture of 

 equal parts of alcohol and ether, and drop them one by one into a 

 jar of alcohol. When required for use they are removed from the 

 alcohol and carefully dried and polished with a clean soft 

 cambric rag. 



Preparation of the film— 



The staining of the film when milk is under examination is 

 described below, and in examining cultural growth in bouillon or 

 other liquid media of a like nature it will be found advisable after 

 fixing to clear the film with a 5 per cent, solution of acetic acid 



