SUPPORTS

24]

Relative
Coefficient of

Hardness
Thermal Expansion *
Specific

of the
(Approximate)
Gravity

Elements

Copper
3.0
i4Xio-6
8.9

Brass (66 Cu, 34 Zn)
—
i9Xio-6
8.4

Iron
4-5
I2XlO~6
7.8

Alu nil num.
2.9
24Xio~6
2.7

* The increase in length per unit length (measured at o° C.) per degree centigrade.
Mortar (see also Piaster) is a building material prepared by mixing lime, plaster
of Paris or hydraulic cement with water and with sand and fibrous materials,
giving a mixture which sets by hydration or carbonation. It is used in masonry
and in plaster. The term * mortar ' is ordinarily restricted to the mixture of the
components of plaster while they are still in the plastic condition after the addition
of water. The term is also applied to any one of these mixtures when used as a
cementing or bonding material for stone and bricks.
Mulberry or Paper Mulberry. The fibres of the paper mulberry (Broussonetia
papyrifera) are used by the Chinese and Japanese in the preparation ;iof certain
special papers (see also Paper). They are separated from the inner bark of the
tree by scraping, soaking, and maceration in water, and they may be purified
still further by boiling in weak alkaline solutions. As used in Oriental papers, the
fibres are generally unbroken. They are long and slender and they vary in length
from 6 to 20 mm., with an average width of 0.030 mm. They are nearly trans-
parent under the microscope and show transverse jointings as well as longitudinal
striae. The central canal generally shows as a well defined line and the ends are
sometimes blunt and rounded, sometimes fringed (see also Fibrous Substances).
Oak (Quercus). The principal European species of oak are Q.fedunculata and
0. sessiliftora^ the two principal species in North America are the white oak
(Q. alba) and red oak (Q. borealis). Oak is a hard wood, grayish brown in the
heart-wood and white in the sap-wood. The red oak varieties have a reddish tinge
in the heart-wood. The figure is produced by the strongly marked annual rings in
tangential section and consists of straight lines, irregular curveSj and parabolas.
In quartered section the figure is produced chiefly by the large medullary rays
which appear as more or less continuous * flakes * across the grain of the wood.
The annual rings are very distinct in end surfaces, with marked segregation of
very large pores at the beginning of each year's growth and very minute pores
in the denser, outer portions. The rays are also strongly marked, being of two types
—large (regular and conspicuous) and very small. The size and distribution of the
large rays make it easy to distinguish oaks from all other woods. Most oak-wood