Page:Popular Science Monthly Volume 29.djvu/632

614 absorption of moisture. Woods impregnated with the heavy tar or lighter oils are protected more from the fact of prevention of access of dampness to the fibers than by the contained antiseptics, unless in the exception of a great percentage of creosote. In the second method the moisture is permitted to come in contact with the fibers of the wood, and reliance depends upon the antiseptic. In this case, the entire wood should be saturated to give the greatest measure of success, not merely an exterior protection of a half-inch or so in depth, the latter fact, as before explained, being the cause of many of the failures which have taken place. The antiseptic treatment, to succeed, must destroy all the germs which have found lodgment in the timber, and also those which may come from the exterior.

In a general paper I can only indicate the antiseptics which have been fairly successful, though in many cases the failures were due not so much to the antiseptic used as to the faulty manner of application, which can be understood from what has been written.

The four antiseptics which are most used now are chloride of zinc, creosote, corrosive sublimate, and sulphate of copper; sulphate of iron and pyrolignite of iron may be mentioned. The treatment of the wood by bichloride of mercury (corrosive sublimate) was called kyanizing; by chloride of zinc, Burnettizing; by creosote, creosoting or the Bethel process; by sulphate of copper, Boucherie's process. Sulphate of copper has been used for over a century in preserving timber, and when well applied the results have been good. The idea of Boucherie was to force the antiseptic through all the wood-cells, which was correct, and the method successful in proportion to the extent it was accomplished.

The attempts to impregnate wood are made now with nearly all of the antiseptics, in large cylinders capable of sustaining from two hundred to three hundred pounds pressure per square inch, one end of which can be opened and closed for admission and withdrawal of the timber. When the cylinders are filled with the timber they are closed, then steam or heat is applied to vaporize the sap or moisture; after this a partial vacuum is produced and sustained for from six to twelve hours, then the moisture is withdrawn from the cylinders, and the antiseptic is pumped in and raised to a pressure of from one hundred and twenty to one hundred and fifty pounds, which is maintained for from six to twenty-four hours. Porous woods are impregnated quite readily, while the heart-wood of the yellow pine (see Fig. 1) and the white oak (Fig. 3 in August number) are not penetrated so easily, and take longer time. The external pressure may be one hundred and fifty pounds per square inch; yet the hydrostatic pressure in the cavities of the cells, not of an inch in area, is quite small, the impregnation being to a great extent by capillary attraction and absorption through the cell-walls.

It is evident from preceding statements and illustrations that