A very simple method is to cause the formalin, diluted about twice with water to furnish moisture enough, to drop by means of a regulated “separator funnel” on a heated iron plate. The dropping should be so regulated that each drop is vaporized as it falls. The plate must have raised edges, pan-shaped, to prevent the drops rolling off when they first strike the plate. Formaldehyde has no corrosive (except on iron) or bleaching action, and is the most nearly ideal closed space disinfectant today. In disinfecting stations it is made use of in closed sterilizers such as were described under steam disinfection particularly in connection with vacuum apparatus. It is also used in solution as a preservative and as a disinfectant. The commonest strength is 2 or 3 per cent. of formalin or 0.8 to 1.2 per cent. of the formaldehyde gas. As an _antiseptic_ it is efficient in dilutions as high as 1 to 2000 of the gas. It is very irritant to mucous membranes of most individuals. =Anilin Dyes.=--Some of the anilin dyes show remarkable selective disinfectant and antiseptic action on certain kinds of bacteria with little effect on others. This has been well shown by Churchman in his work on Gentian Violet. This dye inhibits the growth of _Gram positive_ organisms up to a dilution of one part in 300,000 while for _Gram negative_ organisms it is without effect even in saturated solution. This is nicely shown in the accompanying illustration. This inhibiting effect of anilin dyes is taken advantage of in several methods of isolating bacteria (Chapter XVIII). [Illustration: FIG. 110.--The lower half of the plate is plain agar medium, the upper half the same medium plus gentian violet to make one part in 300,000. The Gram positive organism is on the right and the Gram negative on the left. Streak inoculations were made across both media.] In addition to the above-discussed disinfectants a large number of substances, particularly organic, are used in medicine, surgery, dentistry, etc., as more or less strong antiseptics, and the list is a constantly lengthening one. In the laboratory chloroform, H₂O₂, ether and other volatile or easily decomposable substances have been used to sterilize liquids which could not be treated by heat or by filtration. The agent is removed either by slow evaporation or by exhausting the fluid with an air pump. The method is not very satisfactory, nor is absolute sterilization easily accomplished. It is much better to secure such liquids aseptically where possible. CHAPTER XV. DISINFECTION AND STERILIZATION (CONTINUED). CHOICE OF AGENT.