76 PROPERTIES OF WOOD. 



differently from unfrozen wood, and this proves that water 

 does not exude from the cell- walls of frozen wood. On the 

 contrary, owing to the retention of water by these walls, a 

 reduction of temperature acts oppositely on frozen and un- 

 frozen wood. When the temperature is reduced the latter 

 absorbs water from the air and expands, while frozen wood 

 contracts ; if the temperature is reduced considerably below 

 freezing-point, frozen wood cracks, as does a mass of ice under 

 similar conditions. Hence, when the air-temperature is very 

 low, frostcracks occur in living trees, but they must not be ex- 

 plained by differences of temperature in the internal and exter- 

 nal zones of wood in the tree, phenomena which Mayr states do 

 not occur in nature.* Greenwood when heated above C. 

 follows the laws of evaporation of water, but when cooled 

 below 0, it follows the laws for reduced temperature in 



solids. 



(b) Movements of the Water m Wood. 



When wood is heated not only the temperature of the sub- 

 stance of the wood is raised, but also that of the air and 

 water it contains. If freshly felled wood or wood saturated 

 with water from lying in it is suddenly heated, much water 

 exudes on to the exposed surfaces of the wood ; if then the tem- 

 perature be reduced the exudation of water ceases, but if the 

 wood be submerged, it absorbs still more water. The water 



* [Internal heat of tree*. A remarkable series of experiments, with a view to 

 ascertaining the variations of temperature in trees, has been conducted by Herr 

 F. Schleichert, of Jena, who publishes the results in the Naturwlt&enschaftliche 

 Wochenschrift. 



Herr Schleichert finds that the general temperature of the interior of trees is 

 dependent upon the temperature of the surrounding air, but is influenced also 

 by other causes, such as the ground temperature, the temperature of the water 

 ascending in the wood, and the temperature of the branches, which are directly 

 heated by the sun's rays. 



The mode of experiment was the following : A hole was bored in the stern of 

 a tree on the north side at a height of 1 metres (nearly 5 ft.) from the ground. 

 In the hole was placed to a depth of 12 centimetres (nearly 5 in.) a thermometer, 

 and sealed up with wax. A second thermometer similar to the first, was 

 fastened to a branch of the tree, so that the air circulated freely round it. 



The temperatures registered by the two instruments were taken at varying 

 intervals during the day and compared. 



The readings of the thermometers for eight days in June, which are published, 

 bring to light a curious phenomenon. While the external temperature showed 

 the usual maxima in the afternoon, the maxima in the interior of the tree 

 were recorded at midnight and the minima at midday. Tr.] 



