PRESERVATION OF TIMBER. 353 



treatment is an important element in the success of the creo- 

 soting process. Woody fibre itself is heavier than water; its 

 specific gravity being generally considered as equal to 1/5. It 

 is therefore owing to the looseness of the texture that so many 

 kinds of timber are lighter than water. As fir timber can under 

 certain conditions absorb so much moisture as to become water- 

 logged, or actually heavier than water, its powers of absorption 

 may be estimated from its specific gravity. 1 



The weight of water in fresh-filled timber varies in different 

 species from one-fifth to one-half the total weight. A large 

 quantity of this is rapidly given off by evaporation, but as 

 equality approaches between the hygroscopic state of the atmo- 

 sphere and the moisture of the wood, drying takes place very 

 slowly, and can never be complete. 2 Timber thoroughly air- 

 dried hardly ever contains less than from 15 to 20 per cent, of 

 water after from two to three years' stacking. 3 



Theoretically, timber should be thoroughly dry and free 

 from all sap before being placed in the cylinder. Practically, 

 except perhaps in very special instances, this condition is never 

 attained. Various means have been tried to avoid the loss of 

 time involved in stacking for the purpose of drying and season- 

 ing, such as the application of superheated steam, currents of 

 hot air, drying-stoves or ovens, but none have proved altogether 

 satisfactory, and have been abandoned sooner or later so far as 

 regards the preparation of timber on a large scale for marine 

 purposes. 



The action of the exhaust air-pump, as used in the ordinary 

 process, withdraws the air from the pores of the wood; but 

 however perfect the vacuum may be, it does not extract to any 

 great extent the moisture and sap, upon the absence of which 

 the efficiency of the creosoting process depends. 



By the application of dry heat to the cylinder, the water 

 and sap would be volatilized, and would be withdrawn as steam 

 by the action of the air-pump, but the wood would crack and 

 open to an extent that would be undesirable. 4 



Mr. Boulton meets the difficulty, and gets rid of the moisture 

 in the wood, after it is placed in the cylinder, by taking advan- 

 tage of the fact that while the boiling point of water is 212 

 Fahr., the boiling point of the creosote oils ranges from 380 to 



1 M.P.I.C.E., vol. Ixxviii. p. 124. * Ibid., voL lii p. 293. 



1 Ibid,, Irxviii. p. 125. 4 Ibid. 



2A 



