METHODS OF OBSERVING BACTERIA. 95 ready to cut. The knife must be kept flooded with alco- hol while cutting. II. Paraffin— 12-24 hotirs in 95 per cent, alcohol, 6-12 u u absolute alcohol, 4 " " chloroform, benzole, or xylol, 4-8 " "a saturated solution of paraffin in one of the above reagents, *—' Place in melted paraffin in an oven or paraffin water- bath, at 40°-45° C., until the volatile reagent is all evap- orated, and the tissue impregnated with paraffin. Im- bed in freshly melted paraffin in any convenient mould. In cutting, the knife must be perfectly dry. When it is necessary, subsequently, to remove the im- bedding material, dissolve the paraffin in chloroform, benzole, xylol, oil of turpentine, etc., which in turn can be removed with 95 per cent, alcohol. Celloidin is soluble in absolute alcohol, ether, and oil of cloves. It is very convenient to fasten the cut sections upon the slide—paraffin sections by oil of cloves and collodion or gum arabic solution, celloidin sections by firmly pressing filter paper upon them and rubbing hard, then allowing a little vapor of ether to run upon them. ^ III. Glycerin-Gelatin.—As the penetration of the tissue by celloidin is attended with lessened staining-qualities of the tubercle bacillus, it has been recommended by Kolle* that the tissue be saturated with a mixture of glycerin, r part; gelatin, 2 parts; and water, 3 parts; cemented to a cork or block of wood, hardened in absolute alcohol and cut as usual for celloidin with a knife wet with alcohol. For staining bacteria (other than the tubercle and lepra bacilli) in tissue, two universal methods can be recommended: Loffler's Method.—The cut sections of tissue are stained for a few minutes in Loffler's alkaline methylene- blue solution (q. v.\ and then differentiated in a i per 1 Fliigge's Mikrodrganismen.