Disinfectants are divided into two classes: (1) solid materials used in suspension, or in watery solutions; (2) gaseous substances. The latter are preferable for room disinfection when their use is permissible, for the gas penetrates to every part of the space, even into the cracks. Gaseous disinfectants can only be used when the space is tightly closed, for the gas must be confined for several hours in the room, in order to make the process effective. Such disinfectants can often be used to advantage in the treatment of refrigerators and cheese rooms to destroy mold spores. In less tightly closed spaces, reliance must be placed on the use of the solid or liquid disinfectants. =Lime.= Quick lime or stone lime has a considerable disinfecting action. On exposure to the air, quick lime becomes air slaked, and then has no disinfecting action whatever. Water-slaked lime used in the form of white wash, lime water, or the powder is effective. Air-slaked and water-slaked lime are similar in appearance, but a difference can be noted by placing a particle of each on the tongue; the air-slaked tastes like chalk while the water-slaked material causes the tongue to burn. White wash is one of the most effective agents that can be used in the disinfection of barns, milkrooms, etc. Besides being a fairly strong disinfectant, it has a tendency to absorb odor, it encrusts the walls and lightens the interior of rooms. It can be applied with a brush or with a spray pump. =Carbolic acid and cresol compounds.= These substances are among the cheapest and best disinfectants, but their use in the dairy is not advisable, on account of the penetrating and lasting odor. They can be used to advantage on the farm. Some of the proprietary compounds, as Zenoleum, Kresol, etc., are easily applied, since they mix readily with water in all proportions, forming a milky-white emulsion that can be easily applied. They are less caustic and less poisonous than carbolic acid. =Corrosive sublimate.= Corrosive sublimate is the most efficient disinfectant under ordinary conditions. It is such an intense poison that it must be used with caution in places to which stock have access, or in the dairy. A solution of one part of the salt to a thousand parts of water (half ounce to 4 gallons of water) is the standard generally used. For gutters, drains, and waste pipes in factories, ferrous sulphate (green vitriol), and copper sulphate (blue vitriol), can be used to advantage. They are to be classed as deodorants rather than as true disinfectants. Since they have no odor of their own, they can be used in any amount in the dairy.