Any liquid which boils at a _lower_ degree can be made to boil if immersed in a liquid which boils at a higher degree. Thus a _cup of ether_ can be made to boil in a saucepan of _water_. A _cup of water_ in a saucepan of _brine or syrup_. But a _cup of water_ will _not_ boil if immersed in _ether_; nor a _cup of syrup_ in _water_.

Q. _Why are CLOUDS HIGHER on a FINE DAY?_

A. 1st--Because the _air_ (expanded by heat) _drives them higher up_: and

2ndly--The _clouds themselves are lighter_, and therefore _more buoyant_.

Q. _Why are the CLOUDS LIGHTER on a FINE DAY?_

A. Because their mists are either _absorbed by the dry air_, or _vapourized_ by the hot sun.

Q. _Why is a CUP PUT topsy-turvy into a FRUIT-PIE?_

A. Its princ.i.p.al use is to _hold the crust up_, and _prevent it from sinking_, when the cooked fruit gives away under it.

Q. _Does not the cup PREVENT the FRUIT of the pie from BOILING OVER?_

A. No, by no means; it would rather tend to _make it boil over_, than otherwise.

Q. _Why would the cup tend rather to MAKE the FRUIT BOIL OVER?_

A. As soon as the pie is put into the oven, the _air_ in the cup will _begin to expand_, and drive every particle of juice from under it; the pie dish, therefore, will have a cup-full _less room_ to hold its fruit, than if the cup were _taken out_.

Q. _If the juice is driven OUT of the cup, why is the CUP always FULL of JUICE, when the pie is cut up?_

A. Immediately the pie is drawn, the _air_ in the cup begins to _condense again_, and _occupy a smaller s.p.a.ce_; in consequence of which, there is no longer _enough air to fill the cup_, and so _juice_ rushes in _to fill up the deficiency_.

Q. _Why does JUICE rush into the cup, because the cup is NOT FULL of AIR?_

A. As the external air _presses upon the surface of the juice_, it rushes into the cup _un.o.bstructed_; as mercury rises through the tube of a barometer through similar pressure.

CHAPTER X.

EXPANSION FROM HEAT.

_(Continued.)_

Q. _Does heat expand every thing ELSE BESIDES air and water?_

A. Yes; _every_ thing (that man is acquainted with) is expanded by heat.

Q. _Why does a COOPER make his HOOPS RED-HOT, when he puts them on a tub?_

A. 1st--As _iron expands by heat_, the hoops will be _larger_ when they are red-hot; and will, therefore, _fit more easily on the tub_: and

2ndly--As _iron contracts by cold_, the hoops will _shrink_ as they cool down, and _girt the tub with a tighter grasp_.

Q. _Why does a WHEELWRIGHT make his hoops RED-HOT, which he fixes on the NAVE of a WHEEL?_

A. 1st--That they may _fit on more easily_: and

2ndly--That they may _girt the nave more tightly_.

Q. _Why will the wheelwright's HOOP FIT the nave MORE EASILY, because they are made RED-HOT?_

A. As _iron expands by heat_, the hoops will be _larger_ when they are hot; and (being larger) will go on the nave more _easily_.

Q. _Why will the HOOPS, which have been PUT ON HOT, girt the nave more FIRMLY?_

A. As _iron contracts by cold_, the hoops will _shrink as they cool down_; and, therefore, _girt the nave with a tighter grasp_.

Q. _Why does a farrier put the HORSE-SHOE on HOT?_

A. That it may _stick the closer_, when it has contracted by cold.

Q. _Why does a STOVE make a CRACKING NOISE, when a fire is very hot?_

A. The iron stove _expands by heat_, and (as it swells) the parts rub both _against each other_, and _against the bricks around_, driving them further off; and this produces a _cracking_ noise.

Q. _Why does a STOVE make a similar CRACKING NOISE, when a large FIRE is TAKEN DOWN?_

A. The iron stove _contracts again_, as soon as the fire is removed; and (as it shrinks into a smaller s.p.a.ce) the parts _rub against each other again_, and the _bricks are again disturbed_; and this produces a cracking noise.

Q. _Why does the PLASTER round a STOVE CRACK and fall away?_

A. When the fire is lighted, _the iron-work_ (which expands more than the brick-work and plaster) _pushes away the bricks and plaster_: but when the fire is put out, the metal _shrinks_ again, and _leaves the "setting" behind_.

Q. _Why does the PLASTER FALL AWAY?_