EFFECTS OF HEAT. 77 



exudes owing to the expansion of the sap and air contained in 

 the wood, the expansion of the w r oody substance scarcely 

 intervenes. The heating and cooling of the included water 

 and air also proceeds in living trees, where it is an important 

 factor in the ascent and descent of water, as well as in the 

 lateral passage of water into the medullary rays. 



This movement of water proceeds also in converted wood, 

 if it is insufficiently dried. The water is then driven out- 

 wards by the expanding air, along its natural paths towards 

 the external surfaces of the wood, that therefore become wet. 

 The practical effects of this is to favour dry-rot (M 

 , on joists and planks. 



(r) Decomposition of Wood. 



If wood is heated up to 100 C. it first loses all its water 

 and becomes absolutely dry ; several observations have 

 proved that the other properties of wood are thus somewhat 

 altered. The production of absolutely dry wood is of impor- 

 tance in many experiments, which can be carried out only 

 when the disturbing factor, water, is eliminated. If the wood 

 is heated still further, gases art; produced and ignite when 

 in contact with a flame, until finally only ashes remain ; 

 the rest of the wood passes into the air in the form of water- 

 vapour and carbon-dioxide, with a small amount of ammonia. 

 If air be excluded or admitted insufficiently during the 

 heating of wood, the woody tissue is decomposed : 



From lf>() to 280 into water-vapour, acetic acid, formic 

 acid, methyl alcohol, with a brown 

 residue. 



,, 280 to 360 into carbon-dioxide, air monoxyde, 

 marsh-gas, acetyl, ethyl, and a 

 brown residue. 



,, 360 to 430 into marsh-gas and hydrogen, and a 

 thick brown liquid of paraffin, 

 benzol, carbolic acid, with charcoal 

 as a residue. 



,, 430 to 1,500 wood yields the same products as 

 before (360 to 430), no new ones 

 being formed. 



