108 INTRODUCTION TO BOTANY 
and other things. Even in buildings or sailing craft composed 
largely of steel it is therefore found highly desirable to make 
the floors, decks, and much of the interior construction of wood. 
6. Wood is a poor conductor of electricity. This makes it 
far easier to manage electric wiring in houses or other build- 
ings in which the floor joists and most of the interior finish 
are of wood than it is in metal structures. 
7. When properly finished, wood usually has a highly orna- 
mental surface. This makes it possible to give to the interiors 
of rooms, railway cars, and street cars a decorative effect which 
could be obtained with other materials only with much diffi- 
culty and expense. It is not easy to imagine how beautiful 
furniture of moderate price, such as is made from our orna- 
mental woods, could be made from any metal. 
101. Wood as fuel. At present coal is the fuel used in most 
great manufacturing operations, but the world’s coal supply 
is limited and seems likely at no very distant day to become 
exhausted. The wood supply, with suitable care, can be con- 
tinually renewed, and wood will probably always remain, as 
it is now, an important portion of the fuel resources of the 
world. The fuel value of wood depends somewhat upon its 
weight per cubic foot, so that such heavy woods as hickory, 
sugar maple, ash, beech, and most oaks are worth more for 
heating purposes than such light woods as willow, cottonwood 
and other poplars, and most pines and other coniferous woods. 
Charcoal is used a good deal as a smokeless fuel and is the 
main combustible ingredient in gunpowder. 
102. Coniferous woods. Our native woods! are best clas- 
sified into two principal groups — hard woods and soft woods, 
or coniferous woods.? The needle-leaved, or coniferous trees 
of the country furnish more than three quarters of our timber 
supply. 
1“ Timber,’? Bulletin 10, Division of Forestry, U.S. Dept. Agr., 1895. 
2 Some of the needle-leaved, or coniferous, trees, such as the larch and 
the yew, have rather hard wood, and some broad-leaved trees, such as 
willows, poplars, tulip trees, and buckeyes, have soft wood ; but people who 
deal in timber usually speak of the two general classes as explained above. 
