IMPEEGNATION OF TIMBER WITH ANTISEPTIC SUBSTANCES, 237 



antiseptic substance, instead of using a liquid solution. The wood 

 in the chamber remains exposed to the vapour during 6 to 20 hours. 

 The pneumatic method possesses advantages which render it the 

 most practical of all those yet invented. There is no waste of wood 

 in it, and the wood may be in any condition of seasoning. On the 

 other hand, it requires very expensive plant, which places its 

 adoption beyond the reach of small capitalists. 



AeticiiE 3. — The Immeesion Method. 



This method is the simplest of all. The wood, after it has been 

 thoroughly seasoned, is plunged into a bath containing the anti- 

 septic substance. 



The more prolonged the immersion is, the more fully does the 

 wood become impregnated, and hence the more durable does it 

 become ; but it has been found that very long immersion has the 

 effect of rendering the wood brittle, and 24 hours are considered 

 sufficient. No portion of the wood should be allowed to remain 

 outside the liquid, so that light wood must be sufficiently weighted 

 to remain below the surface. The higher the temperature of the 

 liquid is, the more rapid and effective is the impregnation. Small 

 pieces of timber may be boiled in the bath. 



The substances experimented with in this method are chloride 

 of zinc, sulphate of copper, creosote, sulphate of iron, and tar. 

 The first three, being poisonous, cannot come into general use. 

 The strength of the sulphate of iron solution employed is 15 parts 

 of sulphate to 100 of water. Tar has to be maintained at a tem- 

 perature of 143° C. during the immersion. Except in the case of 

 thin pieces of timber, or when immersion is prolonged beyond the 

 usual duration, the antiseptic substance seldom penetrates into every 

 portion of the tissues, and at any rate does not penetrate equally 

 everywhere. This is, however, not always a drawback, as impreg- 

 nation of merely the outside tissues will generally suffice to pre- 

 vent fungoid growth finding an entrance into the interior. 



Aetiole 4. — Painting ovee the stteeace of the wood. 



Oily and resinous substances in a liquefied condition, if brushed 

 thickly over the surface, enter into and fill up sufficiently the 

 outer tissue to increase very considerably the durability of timber, 

 provided cracks extending beyond the impregnated shell do not 

 form. Timber used under complete exposure to atmospheric in- 

 fluences is tarred with excellent results. 



