water contracts. From 4° C. to 100° C. it expands. At 100° C. it begins to assume the form of steam or vapour. The water rises no higher in temperature, yet heat is supplied for some time. It is absorbed by the water changing its state of aggregation. Steam requires 537 2° C. or 967° F. as its latent heat. Calculations on latent heat will be found in Exercise VII.

It has been already stated that all liquids, like water, when entering the vaporous state absorb immense quantities of heat. Place a drop of water and a drop of ether on the skin; the latter will evaporate rapidly, giving a sensation of cold far stronger than any perceived from the evaporation of the drop of water. That cold is produced by evaporation is clearly illustrated by a simple apparatus called the cryophorus, or ice-carrier (Fig. 86). This simple piece of apparatus

Fig. 86.

B

is made of glass; before the bulb B is hermetically closed, the water in A is boiled, and B is closed at the instant the bulbs and pipe are full of the vapour of steam. We may then look upon the interior as a partial vacuum; and, indeed, this may be shown by allowing the water to fall from one end of the instrument to the other, when it will be noticed that the water falls as in a vacuum, giving a sharp sound as if the contents were metallic, because there is no cushion of air to relieve the fall. To use this instrument, place bulb A in a tumbler glass to prevent draughts of air from retarding the desired effect. Let bulb B be placed in a

second tumbler. Around this bulb, in the second tumbler glass, put a freezing mixture, as ice and salt, finely pounded; the vapour in B will be condensed, more vapour will come off from A. At this stage two points must be kept in view. 1st, the vapour is readily produced from A, because there is little or no pressure on the surface of the water. 2nd, the vapour cannot be produced without absorbing a large quantity of heat that becomes latent. This last vapour is again condensed in A, until the heat is so far removed from this bulb that a coating of ice, half an inch thick, may with ease be obtained.

Combustion.-Candle and Gas Flames.-The Diamond.-Animal Heat.-An Ordinary Fire. -When we clearly understand what takes place in the burning of a common candle, all

other examples of combustion are readily understood. Let Fig. 87 represent a candle flame. The flame is supported by the burning of the melted or vaporous tallow, which flows up the wick by capillary attraction. Tallow is chiefly hydrogen and carbon. In part 1 of the flame there is no combustion going on. Place a piece of glass or wire gauze on the top of the flame, press it down to the middle, and look into this part of the flame; it will be found to be hollow. Again, take a piece of glass tubing of small bore, put one end of it in this part of the flame, bend it twice at right angles, and pass the other end nearly to the bottom of a vessel of cold water; it will be found that solid tallow will collect at the bottom of this vessel. Now there is chemical affinity or attractive force at work between the atoms

Fig. 87.

of hydrogen and carbon of the flame and the oxygen of the atmosphere; but this affinity is much stronger between H and O than between C and O. Imagine a kind of struggle here, whereby the H and C separate, the H hastes to effect combination with the O, leaving the C behind. This combination between H and O takes places with such energy that an enormous amount of heat is developed. Next comes the combination of the C with O; but in order to effect this, the laggard C has to traverse part 2 of the flame, where its solid particles are raised to a white heat, and this it is which gives the light. There can be no light without solid matter raised to a high temperature. That part 2 consists of H and O the glass plate before employed will show, by the moisture deposited upon it, which is, of course, H2O, or water. Part 3, the blue shell outside the candle flame, is the portion of the flame due to the combustion of C; that is, its combination with O. piece of glass tubing the CO, (carbonic acid gas) may be collected and passed into lime water, where it will produce the solid white precipitate, carbonate of lime.

With a

The combustion in ordinary flames is never complete ; much of the carbon passes off in the form of soot. This will be demonstrated by holding a sheet of paper, or, better, a plate of glass over the flame. A better supply of O will make the combination more complete, and then the carbon cannot be so obtained. With a blowpipe supply more air to the candle flame; there will only be a slight deposit of soot, and there will be a great increase to the intensity of the flame: The C can effect combination with the O without traversing the heated cone, 2; and, by its combination, adds to the heat generated; whilst the heat from the

action between H and O is not expended upon the C, therefore it is also at liberty. We place a light to the candle to raise the elements to a sufficiently high temperature to effect combination; this is afterwards kept up by the flame itself, so long as the substances last.

A gas flame may be looked upon as an enlarged candle flame. The materials of combustion are chiefly H and C, as before; and the three parts of the flame may be proved to exist as before, and to throw off the same compounds. How necessary, then, it must be in a small room, or in a large room when many lights are used, to provide for the escape of this deleterious gas, CO,.

m

A modification of the gas flame may be obtained with a Bunsen's burner, which is a very simple contrivance for supplying the flame with a large quantity of air. Fig. 88 will assist the student in comprehending its principle. Gas enters by pipe m, holes at the base of pipe n admit the air, and the combustion takes place at the top of n; the flame gives out intense heat, and little or no light. By stopping the holes at the base of n, it will be at once seen that an ordinary gas flame is produced, and that incomplete combustion is giving off, again, carbon in large quantities.

Fig. 88.

In a coal or coke fire, at the bottom of the fire C combines with O, producing CO2; in the middle of the fire CO2+C gives CO, or carbonic oxide, because here there is a scanty supply of O; towards the top of

*The air supplied mixes with the gas intimately, and when combustion commences, all the H and C can at once enter into combination with O, giving intense heat but no light.

L

the fire, there is more O again, and CO+O gives CO2, which burns with a blue lambent flame.

This combustion is constantly going on in our own bodies; the carbon which we take as food combines with the oxygen of the air we breathe, and we throw off CO2 This may be proved by breathing into a tube which contains lime water, when insoluble carbonate of lime will be produced, giving a milky appearance to the water.

The diamond, Newton declared long ago, was composed of some combustible matter. We know that it is pure carbon, or graphite, and that when burnt in oxygen the production is simply CO,, nothing else. Here, by the help of the mechanical theory, we can form perhaps a clear idea of what is actually taking place when the diamond is burning in O; there is chemical affinity or attractive force between the C and O. The little atoms of O from all sides rush to the C, their motion is stopped, and it is translated into heat; hence the intense white heat seen, and the star-like appearance produced by the combustion of the diamond.

Diathermancy. This term is applied to a property possessed by some substances of transmitting the heat rays which strike upon their surface. It is analogous to the term "transparency" as employed when speaking of glass. A substance which will not permit the heat rays to pass through may be termed "athermanous."

For experiment use plates of glass and rock salt. Place the glass between your face and a hot fire; the light will pass through the glass, but the heat will not. Glass is a suitable material of which to make fire-screens. Now put the plate of rock salt between your face and the fire, and a remarkable difference will be noticed, the heat rays will pass through and strike your face as