About the close of the eighteenth century an American, Count Rumford, who was boring some cannon for the Bavarian government, showed that the amount of heat developed seemed to be entirely dependent upon the amount of grinding or mechanical energy expended. The old theory of a fluid prevailed however until about the middle of the nineteenth century, when a great English experimenter by the name of Joule showed conclusively that the amount of heat developed was due entirely to the amount of mechanical energy which apparently disappeared into the heated body.
Every kind of matter is now believed to consist of little particles, or molecules, which are constantly moving about hitting and bumping against each other in the spaces which exist between them. The fact that minute invisible particles may be given off by a substance is readily shown by opening a bottle of ammonia or exposing a piece of musk in a room. Soon in every part of the room the presence of these substances can be recognized by the odor. Yet nothing can in any possible way be seen to have been added to the air.
The molecules are too small to be seen by the most powerful microscope. There are millions of them in a particle of matter as bigas the head of a pin. When a substance is heated the molecules move more rapidly and strike each other harder. This causes the substance to expand. Heat is a form of energy due to the motion of these molecules. If a condition could be reached where there was no molecular motion, there would be no heat. The effect of heat in causing expansion of gases, liquids and solids has been shown in the preceding experiments.
25.Measurement of Temperature. -From the experiments ithas been seen that gases, liquids and solids expand when heated and contract when cooled. It has been found that most substances expand uniformly through ordinary ranges of temperature, so that if this expansion or contraction is measured, we are able to determine the change of temperature.
Experiment 21. -Slightly warm the bulb of an air thermometer tube and place the open end in a beaker half filled with inky water. Allow the bulb to cool. The tube will become partly filled with the water. When the bulb has become entirely cooled mark the end of the water column with a rubber band. Grasp the bulb with the hand, thus warming the air in it. The water column will run partially out of the tube back into the beaker. Cool the bulb with a piece of ice or a damp cloth. The water will come farther up in the tube than it did when simply exposed to the air. We have here an apparatus for telling the relative temperatures of bodies.
Fig. 24.
Instruments arranged to show the amount of the expansion or contraction of certain materials due to changes in their temperature are called thermometers. These may be gas, liquid or metal thermometers. There must be some uniform temperatures between which the expansion shall be measured if we are to have a basis of comparison. These definite points have been taken as the freezing and boiling points of water at sea level.
Experiment 22. -Fill a four-inch ignition tube with mercury and insert a one-hole rubber stopper having a straight glass tube extending through itFig. 25.
and about 20 cm. above it. It may be necessary to cover the stopper with vaseline to keep out air bubbles. When the stopper was inserted the mercury should have risen a few centimeters in the tube. Mark with a rubber band the end of the mercury column. Gently warm the ignition tube. The mercury column rises. Cool the tube and the column falls. We have here a crude thermometer.
The substance whose expansion is most commonly used to measure the degree of temperature is mercury.
This expands noticeably for an increase in temperature and the amount of its expansion can be very readily determined. The ordinary thermometer consists of a glass tube of uniform bore which has a bulb at one end. The bulb and part of the tube are filledwith mercury. The remaining part of the tube is empty, so that the mercury can freely rise or fall. When the temperature rises, the mercury expands and rises, when the temperature falls, the mercury contracts and sinks.
T h e r e a r e t w o k i n d s o f t h e r m o m e t e r s c a l e s commonly used. In one, the point to which the mercury column sinks when submerged in melting ice is marked 32°, and the point to which it rises at sea level when immersed in unconfined steam, the boiling point, is 212°. The distance between the boiling and freezing points is divided into 180 equal parts. Each one of these parts measures a Fahrenheit degree of temperature. This is the common household thermometer of this country and England.
Another kind of thermometer scale, which is used almost exclusively in scientific work and in those countries where the metric system of weights and measures has been adopted, is called the Centigrade.
In this scale, the point at which ice and snow melt isFig. 26.
THERMOGRAPH.
This device makes a continuous record of the temperature for a week at a time.
marked 0 and the point at which water boils, 100. A degree Centigrade then is 1/100 the distance the column expands when heated from freezing to boiling, instead of 1/180 of this distance as in the Fahrenheit scale. There are a number of different designs of thermometers.
Some are for measuring very high, others for measuring very low, temperatures. Thermometers are also constructed so as to be self- recording.
26.The Three States of Matter. -There are three states or con-ditions in which substances exist: solid, liquid and gas. Examples of these are: the solid metal ball, the liquid water, the liquid metal mercu- ry, and the gaseous air. These have already been dealt with experimen- tally. Almost every one knows that water is a liquid, or a solid, ice, or a gas, steam, depending only on the temperature to which it is subjected. It is not so generally known that the state of all other substances de- pends also upon their temperature.