28.The Measurement of Heat.
Experiment 27. -In each of two beakers or tin cups weigh out 100 g. of water. Carefully heat one of the beakers until the water when thoroughly stirred shows a temperature of 90℃. Cool the other beaker till the temperature of the water is 10℃. Pour the water from one beaker into the other, and after thoroughly stirring note the resulting temperature. Use a chemical thermometer to determine the temperatures.
Weigh out 100 g. of fine No. 10 shot in a tin cup and 100 g. of water inanother. Place the cup containing the shot in boiling water and allow it to remain, stirring the shot occasionally, until its temperature is 90℃. Cool the water in the other beaker until its temperature is 10℃. Determine the temperatures exactly and then pour the shot into the water. After thoroughly stirring determine the temperature of the mixture. Which has the highest temperature, the mixture of water and water or the mixture of shot and water?
Since heat plays such an important part in the activities of the earth we need to know how to measure it. There is a great difference between temperature and the amount of heat. The amount of heat in a spoonful of water at 100 ° would be very much less than in a pailful of water at 10°. It would require more heat to raise a pond of water a small part of a degree than to raise a kettleful many degrees. That is why large bodies of water, although their temperatures never greatly change, are able to absorb and to give out great amounts of heat.
Not only does the amount of heat necessary to raise the temperature of different quantities of the same substance vary, but the amount of heat necessary to raise the temperature of equal quantities of different substances also varies. If a pound of water and a pound of olive oil were placed side by side in similar dishes on a stove, it would be found that the olive oil increases in temperature about twice as fast as the water, i.e. it takes about twice as much heat to raise water as it does to raise the same weight of olive oil one degree. In fact, it takes more heat to raise a given weight of water one degree than it does to raise the same weight of almost any other known substance.
In Experiment 27, the resulting temperature from the water mixture was much higher than from the shot mixture. The shot has much less capacity for heat. The quantity of heat required to raise the temperature of a certain mass of a substance one degree compared to the quantity of heat required to raise the same mass of water one degree is called the specifie heat of that substance. The specific heat of olive oil is .47, of shot .03. That is it takes .47 as much heat to raise a given mass of olive oil and .03 as much heat to raise a given mass of shot one degree as it does to raise corresponding masses of water one degree. In order to compare different quantities of heat, physicists have taken as the unit of measure the quantity of heat required to raise the temperature of one gram of water through one degree C. This unit is called a calorie.
Fig. 31.
29.Light. -The sun is not only the source of
almost all the heat of the earth but also of its light. We have developed artificial self-luminous bodies such as candles, lamps, electric lights, but none of these compares with the light given by the sun. The stars also furnish a little light. Most of the bodies that we know are dark and non-luminous. Some- times some of these which have polished surfaces reflect the light from a luminous body and thus ap- pear themselves to be furnishing light.
An example of this is often seen about sundownwhen the sunlight is reflected from the windows of a house, making them look as if there were a source of light behind them. Any dark body whose surface reflects light appears itself to be luminous as long as the source of light remains, but grows dark again when the source is removed. This is the case of the moon. At new moon, the moon is so situated with respect to the sun, that light is not reflected to the earth and we cannot see it. At full moon, half of the moon"s entire surface reflects the sunlight, and it appears very bright.
30.Direction of Light Movement.
Experiment 28. -Point the pinhole end of a camera obscura or pinhole camera (this consists of two telescoping boxes, the larger having a pinhole atthe end and the smaller a ground glass
plate) at some object and move the ground glass plate back and forth until a sharp image of the object is formed. Sketch on a piece of paper the objectFig. 32.
and the image, showing the direction in
which you think the rays of light must have traveled through the pinhole to form the image.
A photographic camera is constructed in the same way as this little camera, only a lens is placed behind the pinhole to intensify the image, and it is possible to exchange the ground glass plate for a photographic plate.
There are certain properties of light which seem readily apparent from our daily experiences. We cannot see objects in the dark, but if a light is brought into the room so that it can shine upon them, they become visible. We see them because the light is reflected to us from them. All objects except self-luminous bodies are seen by reflected light.
If a candle is held in front of a mirror and we look into the mirror, we see the candle behind it. We know that the candle is not there but that its light is reflected by the mirror in such a way as to make it appear to come from behind the mirror. We see the candle by the light the mirror reflects.
If we wish to see whether the edge of a board is straight, we sight along it. If we wish to hit an object with a bullet, we bring the rifle barrel into our line of sight.
We therefore feel confident that if light is
A LAKE MIRROR.
traveling through a uniform medium, such as air usually is, it goes in a straight line.
Experiment 29. -Place a penny in the center of a five-pint tin pan resting on a table. Stand just far enough away so thatthe outer edge of the penny can be seen over the edge ofthe pan. Have some one slowly fill the pan with water. How is the visibility of the penny affected?
