50 u00b0 f to about 200 u00b0 f generally when employing higher temperatures shorter contact times are employed and conversely when employing relatively low temperatures longer contact times can be employed furthermore the concentration of the organosilicon halide in the atmosphere through which the cellulosic material is passed can be varied up to the saturation level of the atmosphere for the organosilicon halide and if employed a solvent for the organosilicon halide the concentration of the organosilicon halide can range up to the saturation level of the atmosphere but should not be so low as to require excessive contact times in order to effect the desired reaction and to render the cellulosic material water repellent typically the concentration of the organosilicon halide ranges from about 2 volume percent up to the saturation level of the atmosphere within the reacton zone the reaction zone is normally maintained under a slight negative pressure during the treating operation but the pressure therein can vary widely say from as low as 1 torr up to about 760 torr when a cellulosic material having a moisture content of below 2 weight percent is employed pursuant to one aspect of the present invention the contact time must be increased and the variables of organosilicon halide vapor temperature concentration and the contact chamber pressure are maintained so that the final ph of the cellulosic material rendered water repellent does not fall below 2 5 and preferably not below 3 5 the range of conditions which will ordinarily be employed will fall in the following ranges again within the ranges set forth conditions are selected and maintained so that the final ph of the cellulosic material does not fall below 2 5.