and the same effect takes place in a less degree by the evaporation of
water. Seeing, then, that evaporation from so small a surface can
lower temperature so many degrees, consider what must be the effect of
evaporation from the extensive surfaces of walls inclosing our houses.
This experiment (thermometer with bulb inclosed in linen) enables me as
well to illustrate that curious law of nature which necessitates the
introduction of a damp course in the walls of our buildings; it is known
as capillary or molecular attraction, and breaks through that more
powerful law of gravitation, which in a general way compels fluids to
find their own level. You will notice that the piece of linen over the
bulb of the thermometer, having been first moistened, continues moist,
although only its lower end is in water, the latter being drawn up by
capillary attraction; or we have here an illustration more to the point:
a brick which simply stands with its lower end in water, and you can
plainly see how the damp has risen.

From these illustrations you will see how necessary it is that the brick
and stone used for outer walls should be as far as possible impervious
to wet; but more than that, it is necessary the jointing should be
non-absorbent, and the less porous the stone or brick, the better able
must the jointing be to keep out wet, for this reason, that when rain is
beating against a wall, it either runs down or becomes absorbed. If both
brick and mortar, or stone and mortar be porous, it becomes absorbed; if
all are non-porous, it runs down until it finds a projection, and then
drops off; but if the brick or stone is non-porous, and the mortar
porous, the wet runs down the brick or stone until it arrives at the
joint, and is then sucked inward. It being almost impossible to obtain
materials quite waterproof, suitable for external walls, other means
must be employed for keeping our homes dry and comfortable. Well built
hollow walls are good. Stone walls, unless very thick, should be lined