PRESERVATION OF TIMBER. 357 



the operation. The wood used is principally oak and beech. 

 The oak sleepers absorb from 2*4 to 27 quarts per cubic foot, 

 beech sleepers from 87 to 10 quarts per cubic foot. The whole 

 operation takes about four hours. After fifteen years' service, 

 the sleepers taken out have been 15 per cent, for creosoted oak 

 and 50 per cent, for creosoted beech. 1 



The following are extracts from a specification for the 

 preservation of timber sleepers used on New Zealand rail- 

 ways. 2 



" The creosote used under this contract shall be obtained 

 from coal-tar. It is to be perfectly liquid at a temperature 

 of 100 Fahr. It shall contain not less than 20 or more than 

 30 per cent, of constituents that do not distil at a temperature 

 of 600 Fahr., and shall yield not less than 8 per cent of tar 

 acids. 



" The operation of extracting the moisture from the sleepers 

 and impregnating them with creosote shall be as follows : 



" The sleepers shall be put into the creosoting-chamber in a 

 green state. The chamber shall then be filled with creosote to 

 the level of the top of the sleepers, space for gases being left 

 in the dome. The temperature of the creosote is then to be 

 raised to such a point as will exactly convert the moisture in 

 the sleepers into steam without boiling the creosote, but never 

 to exceed 250 Fahr. The steam from the sleepers is to be 

 exhausted by an air-pump connected with the dome, and con- 

 densed into a tank, where it can be measured. 



" The air-pump is to be set to work as soon as the heat is 

 applied, and kept going till all the moisture in the sleepers is 

 extracted. During this time the heat of the creosote is to be 

 carefully regulated to meet the variations in the boiling point 

 through the action of the air-pumps. 



" After the moisture has been completely extracted from the 

 sleepers by the process just described, the chamber is to be 

 filled quite full with creosote, and pressure shall be applied by 

 hydraulic-pumps for such a length of time, and with such force 

 as may be found necessary, to insure that each and every sleeper 

 is impregnated with at least 18 Ibs. weight of creosote. 



" The temperature of the creosote shall be maintained at 1 20 

 Fahr. during the impregnating process. The pressure-pumps 



1 Extract from Revue Ge'ne'rale des Chemins de Per. 

 M.r.LC.K, voL xcui. p. 410. 



