all converted into steam at 212°, what would take place if immediately condensed? What has become of all the heat required to convert the water into vapour, and how would it show itself when the steam is condensed?

If the steam were heated above 212°, how is its expansive force increased? Simply as the temperature, or in a higher ratio ?*

Why, when a mass of ice is dissolved from the heat of a room, or in a vessel on the fire, does the temperature of the water not rise, so long as any ice remains undissolved-(test this by placing a thermometer in melting ice), and why does it rise as soon as it is all melted?

Water being kept perfectly still, may be cooled many degrees below the freezing point, but if shaken, ice would immediately be formed. The extent to which it freezes at once when shaken depends upon this, whether the quantity of heat given out on freezing is sufficient to raise the temperature of the rest higher than 32°. If, for instance, the mass is cooled to 10° below the freezing point, then only 4th is immediately frozen, and in becoming solid it has given out sufficient heat to raise the temperature of the rest up to the point of freezing.

The circumstance of water, when cooled below 39° of Fahrenheit, expanding when further reduced in temperature, should be noticed-this is shown from ice being lighter than water-from the bursting of water-pipes when frozen.

How beautiful the design of Providence in this arrangement, that when the surface water is near the freezing point, being lighter than that which is underneath, it cannot sink. If it had followed the general law, rivers would begin to freeze from the bottom, and become a solid mass of ice-fish and all the other inhabitants of the water would be destroyed: ice also a bad conductor.

Why can the human body bear to be brought in contact with air at a much higher temperature than with a fluid-with a fluid than with a solid, such as hot iron?

A fluid boils, when its temperature is raised to such a point that the elasticity of its vapour is sufficient to overcome the pressure which is acting upon it; whether from the cohesive

*The disruption of vegetable substances produced by the passage of the electric fluid through a tree is caused by the intensity of the momentary heat converting the fluid of the wood into steam.

ness of the substance itself, the pressure of the atmosphere, or any other artificial pressure.

This explains the principle of a vessel called Papin's Digester, made to extract all the nutritive matter from bones. It is a cylindrical vessel, capable of resisting great pressure; closed by a stopcock, which will resist a pressure of many atmospheres. Of course, in this, water may be heated far above the ordinary boiling point, and from its greater heat, most animal substances are made to dissolve.

The boiling point is not changed by bodies mechanically mixed in a fluid-as sand in water; but it is by all those chemically united with it. All soluble salts retard the boiling point of water.*

The processes in the arts and manufactures carried on by distillation and evaporation should be noticed. The continual evaporation going on at all temperatures from every part of the surface of the globe-land and water, animal and vegetable-increasing the transparency of the atmosphere, sometimes when most charged with vapour it is most transparentat others forming clouds, descending in rain to supply our rivers and springs, and to sustain the whole animal and vegetable world.

Formation of vapour absorbs heat, and therefore produces cold-instance a wet towel applied to the temples in case of headache-sometimes wrapped round a bottle containing anything which requires to be cooled-damping the mats in a doorway-a damp bed a very dangerous thing, for want of exercise to generate heat in the body, so as to counteract the cold in drying, &c. That evaporation produced cold had been known in warm climates from an early period, but this had escaped notice in the more temperate ones, until after the invention of the thermometer, when it was soon perceived that on the bulb being wetted, the mercury immediately fell in the stem.

The following may be taken as a way of applying this knowledge to the teaching of children:

Sugar from the sugar-cane. The juices are pressed out by

* The reader will see some interesting tables on the freezing and boiling points of liquids, &c., on the melting points of solids, such as fat, metals, &c., at the end of the volume on Heat in Lardner's Cyclopædia; as also on their expansions at different temperatures. See Appendix (B).

passing the cane between heavy rollers; this contains, besides sugar, a great deal of water—the water is driven off by boil ing-will go away slowly by evaporation.

in

A current of air over anything that is wet takes the moisture up vapour, as it passes over the surface; this changing the wet upon anything into vapour is called evaporation, and produces cold;-dip your finger in water, when there is so little wind that you do not know from what quarter it comes, and you will find the finger colder on one side than the other, this is the side on which the wind blows, and it is colder because there is a greater evaporation on that side of the finger than on the other. The sailor knows this, and when he is becalmed at sea, and does not know from what quarter the wind blows, he wets his finger in his mouth, and holds it up to the air, the cold side is the wind side.

After a shower of rain on your clothes, and whilst they are drying on your back, do you not feel much colder than you did before?-this is the cold arising from the wet on your clothes becoming vapour-and for this reason you should not sit in your wet clothes after you get home.

Why does your ink get thick by standing in the inkstand? This, after what you have heard, you can answer yourselves.

In cold weather you will sometimes observe a quantity of water collected at the bottom of the panes of glass in a roomyou recollect warm air holds up more vapour than cold-the warm air of the room coming in contact with the glass, which is cold from being in contact with the cold air of the atmosphere, is immediately made cooler; this causes the vapour in it to condense on the surface of the glass-become waterit then runs down, and collects in large drops on the wood.

With the aid of a sectional model of the steam-engine, and knowing something of the elastic power of vapour-that its force of elasticity increases in a much higher ratio than that of its temperature-that when reduced below a certain temperature it is immediately condensed-the teacher would be able to explain many of the more important parts of the machine, showing how steam may be adapted to the purposes of man as a moving power.

He would explain how the steam enters alternately below and above the piston rod, and is carried off-by its elasticity giving an up and down motion to the large beam which sets

the machinery in motion-pointing out the parallel motion at the end of the beam, causing the piston rod always to move in the same vertical plane-the up and down motion of the beam causing two dead points, one at its highest, the other at the lowest point of its motion-how the contrivance of a flywheel, by its momentum when once set in motion, carries the machinery over the dead points, &c.

Then again-the importance of having a great quantity of fire surface in the boiler, in order to generate steam rapidlythe saving of fuel by this-the different kinds of boilers in order to effect it-the nature of safety-valves-that a safetyvalve is, in fact, a weak part of the boiler made to give way when the elastic force of the vapour, from increased temperature, becomes so great as to endanger its bursting-the valve opens (or ought to do), at a pressure much below that which would burst the material of which the boiler is made-gauges for measuring the pressure on every square inch of surface at which the engine is working-nature of an atmospheric safetyvalve opening inwards, and why wanted, &c.; that if the steam inside the boiler is suddenly condensed, the boiler would have a tendency to collapse, and an atmospheric valve would guard against this.

Again, when the water in the boiler is very low, the fire surface of the boiler above the water would become heated in a very high degree; danger from this, in an engine not stationary, as in a steam-boat, of the water being thrown over the heated surface, and all converted into steam, and an explosion take place-not perhaps immediately, but after the heated surface was cooled down to a certain temperature.

The boiler of the locomotive steam-engine is of a tubular kind, in order to expose as much surface as possible to the fire; and in this engine, as there can be no fly-wheel to get over the dead points, there are in each machine two engines at work, the dead points of which are at right angles to each other, so that they never occur together.

The following, from Herschel's Discourse on the Study of Natural Philosophy,' will give the reader some idea of these hidden powers of nature when called into action, and show him how much they are perhaps beyond anything he may have been in the habit of imagining them.

"It is well known to modern engineers that there is virtue

in a bushel of coals, properly consumed, to raise seventy millions of pounds weight a foot high. This is actually the average effect of an engine at this moment working in Cornwall. Let us pause a moment and consider what this is equivalent to in matters of practice.

"The ascent of Mont Blanc from the valley of Chamouni is considered, and with justice, as the most toilsome feat that a strong man can execute in two days. The combustion of two pounds of coal would place him on the summit.

"The Menai Bridge consists of a mass of iron, not less than four millions of pounds in weight, suspended at a medium height of about 120 feet above the sea. The consumption of seven bushels of coal would suffice to raise it to the place where it hangs."

It will, perhaps, be difficult to understand the following description of what may be called the mechanical effects of a jet of steam without having recourse to diagrams; but they are curious, and as the same thing may in some measure be tried by a current of air blown or sent rapidly through a hollow tube, this may suggest simple things of an interesting kind.

sup

A jet of steam issuing outwards in any direction, but pose vertically from an orifice, will ascend into the air, with greater or less force, according to its temperature and elasticity, and will by its momentum displace the air which it meets with in its upward course. The jet will be rendered visible by the steam being condensed, and the effect of this jet upon the flame of any burning substance-or any light substances brought near to the axis of it-by its attracting them (a current of air setting in on all sides towards the axis of the jet), is striking and worthy of attention.

Take a piece of tow, dipped in spirits of wine and placed at the end of a rod, set it on fire, and approach the flame near the axis of the steam jet; when held a little above the orifice from which the steam proceeds, the flame will be attracted in a slanting direction, and the angle which the flame makes with the axis of the jet increases as the distance from the orifice increases, up to a certain point, when it becomes a right angle; elevated above this it again assumes the position it had below this point, until it is elevated beyond the influence of the jet, when it of course assumes a vertical position. This is better shown by taking a circular piece of iron,