the joint action of the added temperature and two
or more qualities which the water possesses.

1st. Water, in common with most other substances, expands when heated; a
statement, however, strictly true only when referred to a temperature
above 39 degrees F. or 4 degrees C., but as in the making of steam we
rarely have to do with temperatures so low as that, we may, for our
present purposes, ignore that exception.

2nd. Water is practically a non-conductor of heat, though not entirely
so. If ice-cold water was kept boiling at the surface the heat would not
penetrate sufficiently to begin melting ice at a depth of 3 inches in
less than about two hours. As, therefore, the heated water cannot impart
its heat to its neighboring particles, it remains expanded and rises by
its levity, while colder portions come to be heated in turn, thus
setting up currents in the fluid.

Now, when all the water has been heated to the boiling point
corresponding to the pressure to which it is subjected, each added unit
of heat converts a portion, about 7 grains in weight, into vapor,
greatly increasing its volume; and the mingled steam and water rises
more rapidly still, producing ebullition such as we have noticed in the
kettle. So long as the quantity of heat added to the contents of the
kettle continues practically constant, the conditions remain similar to
those we noticed at first, a tumultuous lifting of the water around the
edges, flowing toward the center and thence downward; if, however, the
fire be quickened, the upward currents interfere with the downward and
the kettle boils over (Fig. 1).

[Illustration: Fig. 1]

If now we put in the kettle a vessel somewhat smaller (Fig. 2) with a
hole in the bottom and supported at a proper distance from the side so
as to separate the upward from the downward currents, we can force the
fires to a very much greater extent without causing the kettle to boil
over, and when we place a deflecting plate so as to guide the rising
column toward the center it will be almost impossible to produce that
effect. This is the invention of Perkins in 1831 and forms the basis of
very many of the arrangements for producing free circulation of the
water in boilers which have been made since that time. It consists in
dividing the currents so that they will not interfere each with the
other.

[Illustration: Fig. 2]

But what is the object of facilitating the circulation of water in
boilers? Why may we not