First, be sure the slide is clean and _free from grease_. This is accomplished most readily by scouring a few minutes with finely ground pumice stone and a little water, then washing and drying with a grease-free cloth, handkerchief, or piece of cheese-cloth. With the “loop” needle place in the middle of the slide a small loop of water. This is best done by filling the loop by dipping in water, then tapping it gently so that all that remains is the water that just fills the loop level full, and this amount is placed on the slide by touching the flat side of the loop to the glass. Then the _straight needle_ is sterilized, dipped into the culture and just touched once into the small drop of water on the slide. The remainder of the culture on the straight needle is then burned off and the needle is used to spread the drop of water containing the bacteria into a thin even film, which will result, provided the slide is free from grease. This is dried and then “fixed” by passing three times through the Bunsen flame at intervals of about one second, passing through slowly for thick slides and a little more rapidly for thin ones. If the culture is in a liquid medium, the use of the loop of water is unnecessary; a loop of the fluid from the surface, middle or bottom as the culture indicates is spread out to a thin film, dried and fixed. After the film is fixed the stain desired is dropped on, allowed to act for the proper time, which will depend on the stain and the preparation, washed in water, dried thoroughly and examined with the oil-immersion lens, without a cover. If it is desired to preserve the preparation it may then be mounted in balsam. This is not necessary, as they keep just as well, provided the immersion oil is removed. To do this, fold a piece of filter paper so that at least three thicknesses result. Lay this on the slide and press firmly several times, when the surplus oil will be taken up by the paper. Slides not mounted in balsam are more apt to become dusty than those that are. This is the only disadvantage. =Gram’s Method of Staining.=--It has been ascertained that some bacteria contain a substance, possibly a protein, which forms a compound with gentian violet and iodine, which compound is insoluble in alcohol, and other bacteria do not contain this substance. Consequently when bacteria are stained by Gram’s method (given below), those that contain this chemical remain colored, while if it is not present the dye is washed out by the alcohol and the bacteria are colorless and may be stained by a contrast stain. The bacteria which stain by this method are said to “take Gram’s” or to be “Gram-positive,” while those that decolorize are called “Gram-negative.” The method is: 1. Prepare the film as above given. 2. Stain with fresh anilin gentian violet 1 minute. 3. Wash in tap water. 4. Cover with Gram’s solution 1 minute. 5. Wash in tap water. 6. Wash with 95 per cent. alcohol three times or until no more color comes out.