126 FAEM BUILDINGS IN SOUTH AFEICA 



the ground. If the above precautions are observed there is little danger 

 of the decay of timber used in the construction of ordinary buildings. 



To prevent the occurrence of dry rot when timber is to be placed in 

 positions favourable to attack, the following methods are applied : — 



1. Creosoting, or impregnating the seasoned timber to a certain 



depth with oil of tar. This may be done by soaking the 

 wood in hot oil of tar for some hours. 



A better method, which secures greater penetration, con- 

 sists of placing the seasoned timber in an iron vessel which 

 closes air-tight. The air is then exhausted from the vessel, 

 so that the timber in it is subjected to the action of a partial 

 vacuum. Next, the creosote is forced in till the pressure 

 in the vessel rises to over 100 lbs. per square inch. The 

 whole process lasts only 2 or 3 hours. At Knysna, sleepers 

 are creosoted in this way in order to increase their durability. 

 Even when the creosote is forced into the pores of the wood 

 under pressure, it does not penetrate to the centre of a 

 sleeper. 



2. A solution of corrosive sublimate (HgCl 2 ) in water — or, better, 



in methylated spirits — in the proportion of 6 ozs. sublimate 

 to 1 gallon water, or spirit, applied to the timber, is con- 

 sidered an effective wash. This wash also makes the wood 

 repugnant to white ants. 

 Treatment of Timber affected by Dry Rot. — If timber is affected by 

 dry rot it should either be wholly removed, or at least affected pieces 

 should be removed, according to the extent to which the disease has 

 spread. Timber which is not removed should be thoroughly painted 

 over several times with the wash of corrosive sublimate already mentioned. 

 All surroundings, such as brickwork, stonework, etc., should be well 

 sprayed with the same solution, special care being devoted to places 

 which were in contact with the decayed timber as, for instance, holes 

 in the walls in which the ends of beams rested. New timber, put in 

 to replace that removed, should be treated either with corrosive sublimate 

 or creosote. 



Besides taking the above precautions, the ventilation should be im- 

 proved as much as possible, e.g. by the insertion of air-bricks in the 

 case of ground floors. 



