bURABILITT COMBOBTIBILITY AND HEATING POWEE. 27 



crumbles to pieces under the slightest pressure ; while the beetles, 

 when they are not out feeding or mating, live in the galleries 

 where they lay their eggs. 



The third and last class of insects comprises almost exclusively 

 the various species of white-ants. A striking instance of the few 

 other families falling under this class is that of a species of Bostry- 

 chid beetle which, until the tree is felled or has begun to die, 

 lives in the thick bark of the Pinus longifolia, but works its way 

 into the wood within a few minutes of the fall of the tree. The 

 only remedy against this beetle is to bark the trees without delay. 

 As regards white-ants, there are certain woods which are self- 

 protected, either because, like teak and deodar, they contain an oil 

 not relished by the insects, or, like Salvadora and nim, they are 

 impregnated with an acrid alkaloid, or because, like Hardwichia 

 binata and khair, they are too hard for them. In the case of other 

 woods nothing short of impregnating them, or painting them over 

 with poisonous substances, will protect them against these all- 

 devouring pests. 



Wood iised for marine purposes is subject to the attacks of 

 certain crustaceans and mollusca, the most terrible of the latter 

 being the barnacle (^Teredo navalis). Against this last the only 

 sure preservative is to plate the wood with iron or copper. In 

 the case of wood kept in dockyards before use, the best plan is to 

 bury it in mud at the bottom of the tanks, or to reduce the saltness 

 of the sea-water by mixing enough fresh water, as a certain de- 

 gree of brackishness is essential for the barnacle. 



Section IX.— Combustibility and Heating Poweb. 



By the term " combustibility " we mean the ease or difficulty 

 with which a substance takes fire, and, being once ignited, continues 

 to burn until it is consumed ; and by the heating power of a wood 

 is undei-stood the quantity of heat radiated by a unit of volume or 

 weight of the wood when burning in the ordinary way. The only 

 two elements of wood which burn are its carbon and hydrogen, 

 the former combining with oxygen to form carbonic acid, the latter 

 to form water ; while the incombustible portions remain behind as 

 ash. It is very probable that the combustibility and heating 

 power of the pure wood fibre is the same for all woods, and that 

 the actual differences existing between the various woods are due 

 entirely to differences of structure and the presence of accidental 

 substances, such as oils, resins, &c. 



Combustibility is in direct proportion to looseness of texture 

 (guaranteeing free access of oxygen into the interior), to absence 



