7. Dry and examine. Gram’s solution is: I 1 part KI 2 parts H₂O 300 „ This method is excellent for differentiating Gram-positive and Gram-negative organisms on the same slide. First stain by this method and after washing with alcohol stain with a counter-stain, carbol-fuchsin diluted ten to fifteen times with water is excellent. The Gram-positive bacteria are violet and the Gram-negative are red. It is also of great value in staining Gram-positive bacteria in tissues, but the sections should be stained about five minutes in the anilin gentian violet and be left about two minutes in the Gram’s solution. Sections are to be counter-stained in Bismarck brown, dilute eosin or safranin solutions and cleared in oil of bergamot, lavender or origanum and not in clove oil or carbol-xylol, as these latter dissolve out the dye from the bacteria. =Staining of Spores in the Rod.=--Prepare the films as usual. Cover with carbol-fuchsin, using plenty of stain so that it will not dry on the slide; heat until vapor arises, not to boiling; cool until the stain becomes cloudy and heat again until the stain clears, and repeat once more; wash in tap water and then wash in 1 per cent. H₂SO₄ three times, dropping on plenty of acid, tilting and running this over the slide three times and then pour off and use fresh acid and repeat this once. Wash thoroughly in _distilled_ water, then stain with Löffler’s blue one to three minutes. Wash, dry and examine. The spores should be bright red in a blue rod. This method will give good results if care is taken to secure cultures of the right age. If the culture is too old the spores will all be free outside the rods, while if too young they will decolorize with the acid. For _Bacillus subtilis_ and _Bacillus anthracis_, cultures on agar slants forty-eight hours in the 37° incubator are just right. For the spores of _Clostridium tetani_, the culture should be three days old, but may be as old as a week. =Staining of “Acid-fast” Bacilli.=--_Mycobacterium tuberculosis, Mycobacterium of Johne’s disease, “grass” and “butter bacilli,” Mycobacterium lepræ, Mycobacterium smegmatis._ _Gabbet’s method_: 1. Prepare the film as usual. 2. Stain with carbol-fuchsin as given above for spores. 3. Wash with tap water. 4. Decolorize and stain at the same time with Gabbet’s blue, two or three minutes. 5. Wash, dry and examine. The sulphuric acid in Gabbet’s blue removes the carbol-fuchsin from everything except the “acid-fast” bacteria, which remain red, and the blue stains the decolorized bacteria and nuclei of any tissue cells present. _Ziehl-Neelson method_: 1, 2, 3, as in Gabbet’s method. 4. Decolorize with 10 per cent. HCl until washing with water shows only a faint pink color left on slide. 5. Wash thoroughly. 6. Stain with Löffler’s blue one or two minutes. 7. Wash, dry and examine. The results are the same as with Gabbet’s method.