We have seen that the ice-lens was not affected by the passage of heat
through it. If we now take hold of the lens we shall experience a
feeling of cold, and the lens will begin to melt. Heat has passed from
our hand into the ice. The process by which heat passes from one body to
another in contact with it is called conduction. The fundamental law of
conduction is, that heat always passes from a warm body to a cold one.
Clerk Maxwell illustrated this law in a series of very simple
experiments. He placed a silver teaspoon in a cup of hot tea, and noted
that the handle became warm gradually from the hot tea; the heat passed
from the bowl of the spoon in the tea to successive parts of the handle
until the whole spoon was hot. His second experiment was to put two cold
spoons, one of silver and one of German silver, into the tea, when he
found that the same phenomenon took place, but that the silver spoon
became hot much more quickly than did the German silver one. He then put
three spoons into the tea, made respectively of silver, of German
silver, and of bone. In the result, he found that when the other two
were hot, the bone spoon hardly showed any sign of heat at the end of
its handle.
The conclusion to be drawn from these experiments is that heat passes at
different rates through different substances. Substances through which
heat passes quickly are called good conductors of heat. The law of the
conductivity of heat is that in a homogeneous body the flow is
continuous, and is from the region of high temperature to the region of
low temperature, and that it continues until the body is of uniform
temperature throughout. The law is the same for bodies of different
materials when in contact one with another.
The conduction of heat is in operation in every department of domestic
life. People live in houses and are clothed to protect them from the
vicissitudes of the weather, including the cold of winter and the heat
of summer; use is made of the phenomenon in warming the house and in the
preparation of food.
In selecting materials for various purposes, account has to be taken of
their conductivities, for in some cases it is desirable that the
transfer of heat should take place slowly, and in others that it should
take place quickly. It might be thought that the conductivity of a
substance could be estimated by touch, but a little reflection will
show that this cannot be the case. The flow of heat between two bodies
depends upon the difference of temperature between them, and if there
should be no difference of temperature between them at the moment of
touch there will be no flow of heat, though both are bodies of greater
or less conductivity. Let us take, for example of the uncertainty of
estimation by touch, a well-known experiment. Suppose we have a basin of
hot water and a basin of cold water, and place a hand in each for a few
moments; suppose we withdraw the hands and plunge them into a basin of
tepid water, we shall find that the tepid water feels cold to the hand
that was in the hot water and warm to the hand that was in the cold
water.
Luckily, it has been found possible in the laboratory to refer
substances to a common standard and to assign numerical values to them
in order of their conductivities, so that substances can be compared and
a selection made for any desired purpose. Pure silver has the highest
conductivity; other useful materials take the following order: copper,
zinc, lead, iron, steel, marble, glass, brick, slate, wood, fur, cotton,
flannel, water, air. Fur and wool no doubt owe much of their warmth to
the fact that they consist of fibres which enclose a good deal of air,
but as a matter of fact the warmth of loosely woven woollen and knitted
articles in general is often overrated; they are very warm as under
garments or in calm weather, but in windy weather the air in them is
rapidly changed and the cold seems to blow through them. If for any
purpose we select a material from its place in a table of comparative
conductivities, and use it without reference to the law of conduction of
heat, we shall probably be disappointed with the result. We know that
cotton burns easily; if we stretch a cotton handkerchief over the back
of a gold watch and place a red-hot cinder from the fire on the
handkerchief on the watch, the handkerchief will not be burnt.
Many interesting problems present themselves when a house has to be
built or rented. There is often opportunity for some choice of material
in walls or roof, and some peculiarities to be considered. Are the top
rooms of a thatched cottage warmer or colder than the top rooms of a
house covered with slates? Is a wooden or an iron building warmer? What
difference does it make if the iron building is lined with wood? If the
iron walls were twice as thick, what would be the effect inside the
room? Would the walls of such a building be always dry inside? It
sometimes happens that the end wall of a row of houses is covered with
slates to preserve it from the effects of storms of wind and rain; will
that inside wall be always dry?
But the housewife is probably more interested in those articles in
use in the house which it is her business to provide. Shall the
stoves be of slate or iron? In olden days warming-pans were made of
copper. What change in the manner of use justifies making them of
earthenware or India-rubber? The slow transmission of heat through thick
woollen materials has been applied to the construction of Norwegian
cooking-stoves (Fig. 5). These stoves consist of a wooden box, lined
with well-padded felt. The cooking vessels are of metal; the food when
at boiling point is placed in these vessels and the lids put on, a
thick padded felt is placed on the vessels and entirely fills the
wooden lid of the box which is then closed; the heat is preserved so
that the cooking is continued without further attention. Would it be
possible to use the Norwegian stove as a refrigerator? Would it keep
an ice pudding cold without any alteration? In connection with this
we may ask why freezing machines have the inner vessel in which the
freezing takes place of zinc, and the outer vessel which contains the
ice and salt of wood? What would be the effect of interchanging the
materials?
[Illustration: FIG. 5.]
It is possible that the excellence of some continental cookery is due to
the extensive use on the continent of earthenware cooking utensils
through which heat passes very slowly. The growing fashion of using
enamelled cooking vessels must have some effect on the food cooked in
them as heat certainly passes quickly through them. Reference has been
made to them simply to demonstrate the universality of the application
of physical laws, and we may now return to the house and its arrangement
for the comfort of the inmates.