of copper and one of iron, with their ends together, at which they are heated by means of a lamp. After the lamp has burned for some time, let us take two little bits of phosphorus, and place one of them at the end of the copper rod furthest away from the flame. It will soon take fire. Now place the other piece on the iron rod at the same distance from the lamp as the burning phosphorus, and it will not take fire. This shows us that the heat of the lamp is conducted more powerfully along the copper than along the iron.

The conduction of heat explains the action of the safety lamp which was devised by Sir Humphry Davy for the use of miners, but this very useful lamp has already been fully described in the Chemica: Primer (Art. 41).

66. Convection of Heat.-If we take a vessel full of water, and float on its surface a vessel full of boiling oil, we shall find that the heat of the oil will be conducted very slowly indeed downwards through

the liquid; in fact, a few inches down, the rise of temperature will be hardly perceptible. But if instead of heating the vessel with water in it from above we heat it from below, as in the figure, we shall find that in a very short time the whole water will be heated and begin to boil. In fact, as we have already stated, the heated particles getting lighter rise, and are replaced by colder and heavier particles from above, so

that we have a current as is shown by the arrows in the figure, the heated water ascending in the middle and the cold water coming down the sides.

We have several good examples of convection in nature; for instance, in a lake which is cooled at its surface by the action of intense cold. The surface particles are first cooled, and getting heavier they sink

down and are replaced by lighter and warmer particles from beneath, so that in a short time the whole body of water becomes cooled down to a temperature about 4° above the freezing-point; after that temperature the water, contrary to the usual practice of things, expands when further cooled instead of contracting; and when ice is formed, this ice; being decidedly lighter than water, floats on the top.

Now, had ice been heavier than water, it would have fallen down to the bottom as it was formed, a fresh surface would thus have been exposed, and the whole lake would soon have become one mass of ice. But as it is, the cold can only freeze the second layer of water through the ice of the first, and this is a very slow process, so that there is no danger of a lake being permanently frozen.

In the air again we have strong convection currents due to heating; for it is on this account that the hot air of a fire goes up the chimney, being replaced by cold air from the room; and we have the very same thing on a large scale in the great system of winds, for at that part of the earth called the equator, where the sun is most powerful, the air when heated mounts up just as the air of a fire mounts up the chimney. This air is then replaced by currents blowing along the surface of the earth from the poles or colder portions of the earth. We have thus at the equator, a system of upward currents which carry off the hot air to the poles in the upper regions of the air, and we have also currents blowing along the surface of the earth, which bring back this air when cooled to the equator. These surface-currents blowing from the poles to the equator are called the trade winds.

67. Radiant Heat and Light.-The third method by which a hot body parts with its heat is by radiation, and it is in virtue of this process that the heat of the sun reaches our earth. We need not, however, go farther than our own firesides to get an example of the process. If we stand opposite a strong fire, we find our faces and our eyes suffering from the heat. Even a kettle containing hot water gives out radiant heat, although the rays of heat from it cannot pierce the eye and impress it with the sense of light like those from the fire or from the sun. Thus when you heat a body such as a ball of clay, something of the following kind takes place. The body begins at once to rise in temperature, and in consequence to give out rays of heat, but those rays are dark rays, and do not affect the eye. As the heating process goes on, a few of the rays which it gives out begin to affect the eye and the body becomes red hot; it next acquires a yellow heat, next a white heat, and last of all it glows with an intense light resembling the sun. Let us now devote ourselves for a short time to the study of these bright rays which a hot body gives out.

68. Velocity of Light.-Römer, a Danish astronomer, was the first to find out the velocity with which light travels through space. To understand what this means let us remember what takes place when a distant gun is fired off. We see a flash, and then some seconds after we hear a report. Evidently then the sound does not reach the ears at the very moment when the gun is fired, because it lags behind the light. But does the light reach us at the very moment? may not both light and sound start from the cannon at the same moment, and each take some time

to get to us, the light winning the race and coming in first? This point can only be decided by observation and experiment, and it was by observation that Römer found it out. There is a large planet called Jupiter, which is sometimes very far from us and sometimes comparatively near, and this large planet has several satellites, or small attendants, one of which passes across the disc or surface of Jupiter at regular intervals, so that when we use a powerful telescope we see the small satellite like a black body crossing the large disc of the planet. Now Römer found that when Jupiter was very far away from us the satellite seemed to be later in crossing than it ought, and he argued from this that we on the earth do not see the crossing of the satellite over the disc or surface of Jupiter at the very moment when it takes place, but that light takes some time to get from Jupiter to our eyes, just as the report of a distant gun takes some time after the explosion to reach our ears.

You thus see that light as well as sound takes time to travel, only light travels much faster than soundlight travels at the enormous rate of 186,000 miles a second, while sound creeps along at the rate of 1,100 feet in the same time. Light only takes 8 minutes to come from the sun to us, although the distance is 90 millions of miles. If, therefore, the sun were to be suddenly extinguished, we should not find it out until 8 minutes afterwards.

Do not, however, suppose that light consists of small particles shot out by hot bodies, and flying through space at the enormous rate of 186,000 miles a second. If this were the case, we should be knocked to pieces by a ray of light. A ray of light may be said