he order of their fitness for
building purposes are as follows: (1) primitive rock; (2) gravel, with
pervious soil; (3) sandstone; (4) limestone; (5) sandstone, with
impervious subsoil; (6) clays and marls; (7) marshy land, and (8) made
soils.

It is very seldom, however, that a soil can be secured having all the
requisites of a healthy site. In smaller places, as well as in cities,
commercial and other reasons frequently compel the acquisition of and
building upon a site not fit for the purpose; it then becomes a
sanitary problem how to remedy the defects and make the soil suitable
for habitation.

=Prevention of the Bad Effects of the Soil on Health.=--The methods
taught by sanitary science to improve a defective soil and to prepare
a healthy site are the following:

(1) Street paving and tree planting.
(2) Proper construction of houses.
(3) Subsoil drainage.

_Street Paving_ serves a double sanitary purpose. It prevents street
refuse and sewage from penetrating the ground and contaminating the
surface soil, and it acts as a barrier to the free ascension of
deleterious ground air.[12]

_Tree Planting_ serves as a factor in absorbing the ground moisture
and in oxidizing organic impurities.

_The Proper Construction of the House_ has for its purpose the
prevention of the entrance of ground moisture and air inside the house
by building the foundations and cellar in such a manner as to entirely
cut off communication between the ground and the dwelling. This is
accomplished by putting under the foundation a solid bed of concrete,
and under the foundation walls damp-proof courses.

The following are the methods recommended by the New York City
Tenement House Department for the water-proofing and damp-proofing of
foundation walls and cellars:

_Water-proofing and Damp-proofing of Foundation Walls._--"There shall
be built in with the foundation walls, at a level of six (6) inches
below the finished floor level, a course of damp-proofing consisting
of not less than two (2) ply of tarred felt (not less than fifteen
(15) pounds weight per one hundred (100) square feet), and one (1) ply
of burlap, laid in alternate layers, having the burlap placed between
the felt, and all laid in hot, heavy coal-tar pitch, or liquid
asphalt, and projecting six (6) inches inside and six (6) inches
outside of the walls.

"There shall be constructed on the outside surface of the walls a
water-proofing lapping on to the dam