application in the condensation of the volatile gases from oil wells into gasoline. Many solids hold, closely adhering to their surfaces, thin layers of the gases with which they are in contact; so that the prominence of the phe

nomena of absorption in porous substances is probably due primarily to the great extent of surface possessed by such substances.

FIG. 110. Absorp

That the same substance exerts widely different attractions upon the molecules of different gases is shown by the fact that charcoal will absorb 90 times its own volume of ammonia gas, 35 times its volume of carbon dioxide, and only 1.7 times its volume of tion of ammonia hydrogen. The usefulness of charcoal as a deodorizer is due to its enormous ability to absorb certain kinds of gases. This property makes it available for use in gas masks. The metal palladium when heated in hydrogen can absorb 800 times its own volume.

gas by charcoal

123. Absorption of gases in liquids. Let a beaker containing cold water be slowly heated. Small bubbles of air will be seen to collect in great numbers upon the walls and to rise through the liquid to the surface. That they are bubbles of air and not of steam is proved, first, by the fact that they appear when the temperature is far below boiling and, secondly, by the fact that they do not condense as they rise into the higher and cooler layers of the water.

The experiment shows two things: first, that water ordinarily contains considerable quantities of air dissolved in it, and, secondly, that the amount of air thus dissolved decreases as the temperature rises. The first point is also proved by the existence of fish life, for fishes obtain from the dissolved air the oxygen needed to support life.

The amount of gas absorbed by water varies greatly with the nature of the gas. At 0° C. and a pressure of 76 centimeters 1 cubic centimeter of water will absorb 1050 cubic centimeters of ammonia, 1.8 cubic centimeters of carbon

dioxide, and only .04 cubic centimeter of oxygen. Commercial aqua ammonia is simply ammonia gas dissolved in water. The following experiment illustrates the absorption of ammonia by water:

Let the flask F (Fig. 111) and tube b be filled with ammonia by passing a current of the gas in at a and out through b. Then let a be corked up and b thrust into G, a flask nearly filled with water colored slightly red by the addition of litmus and a drop or two of acid. As the ammonia is absorbed, the water will slowly rise in b, and as soon as it reaches F it will rush up very rapidly until the upper flask is nearly full. At the same time the color will change from red to blue because of the action of the ammonia upon the litmus.

Experiment shows that in every case of absorption of a gas by a liquid or a solid the quantity of gas absorbed decreases with an increase in temperature - a result which was to have been expected from the kinetic theory, since increasing the molecular velocity must of course increase the difficulty of retaining the gaseous molecules.

a

FIG. 111. Absorption of ammonia

by water

124. Effect of pressure upon absorption. Soda water is ordinary water in which large quantities of carbon dioxide gas have been absorbed. This impregnation is accomplished by bringing the water into contact with the gas under high pressure. As soon as the pressure is relieved, the gas passes rapidly out of solution. This is the cause of the characteristic effervescence of soda water. These facts show clearly that the amount of carbon dioxide absorbed by water is greater for high pressures than for low. As a matter of fact, careful experiments have shown that the amount of any gas absorbed is directly proportional to the pressure; so that if carbon dioxide under a pressure of 10 atmospheres is brought into contact with water, ten times as much of the gas is absorbed

as if it had been under a pressure of 1 atmosphere. This is known as Henry's law.

SUMMARY. The absorbing power of solids and liquids for gases decreases with increase in temperature.

The quantity of gas absorbed by a liquid is directly proportional to the pressure.

QUESTIONS AND PROBLEMS *

1. In the manufacture of incandescent vacuum lamps why are the glass globes made very hot?

2. Why are bubbles of air seen clinging to the inner surface of a tumbler containing cold water? What difference would a rise in the temperature of the room make in the number and size of the bubbles?

3. Why do fishes in a small aquarium without plants die if the water is not frequently renewed?

* Supplementary questions and problems for Chapter VI are given in the Appendix.

(s = datame~)

CHAPTER VII

WORK AND MECHANICAL ENERGY *

DEFINITION AND MEASUREMENT OF WORK

125. Definition of work. Whenever a force moves a body on which it acts, it is said to do work upon that body, and the amount of the work accomplished is measured by the product of the force acting and the distance through which it moves the body. Thus, if 1 gram of mass is lifted 1 centimeter in a vertical direction, 1 gram of force has acted, and the distance through which it has moved the body is 1 centimeter. We say, therefore, that the lifting force has accomplished 1 gram centimeter of work. If the gram of force had lifted the body upon which it acted through 2 centimeters, the work done would have been 2 gram centimeters; if a force of 3 grams had acted and the body had been lifted through 3 centimeters, the work done would have been 9 gram centimeters; etc. Or, in general, if W represents the work accomplished, F the value of the acting force, and s the distance through which its point of application moves, then the definition of work is given by the equation

In the scientific sense no work is ever done unless the force succeeds in producing motion in the body on which it acts. A pillar supporting a building does no work; a man tugging

* It is recommended that this chapter be preceded by an experiment in which the student discovers for himself the law of the lever, that is, the principle of moments (see, for example, Experiment 15 of "Exercises in Laboratory Physics," by Millikan, Gale, and Davis), and that it be accompanied by a study of the principle of work as exemplified in at least one of the other simple machines (see, for example, Experiment 17 of "Exercises in Laboratory Physics," by Millikan, Gale, and Davis).

典

at a stone but failing to move it does no work. In the popular sense we sometimes say that we are doing work when we are simply holding a weight or doing anything else which results in fatigue; but in physics the word "work" is used to describe not the effort put forth but the effect accomplished, as represented in equation (1).

126. Units of work. There are two common units of work in the metric system: the gram centimeter and the kilogram meter. As the names imply, the gram centimeter is the work done by a force of 1 gram when it moves the point on which it acts 1 centimeter. The kilogram meter is the work done by a kilogram of force when it moves the point on which it acts 1 meter. The gram meter also is sometimes used.

Corresponding to the English unit of force, the pound, is the unit of work, the foot pound. This is the work done by a "pound of force" when it moves the point on which it acts 1 foot. Thus, it takes a foot pound of work to lift a pound of mass 1 foot high.

In the absolute system of units the dyne is the unit of force, and the dyne centimeter, or erg, is the corresponding unit of work. The erg is the amount of work done by a force of 1 dyne when it moves the point on which it acts 1 cm. To raise 1 1. of water from the floor to a table 1 m. high would require 1000 × 980 × 100 98,000,000 ergs of work. It will be seen, therefore, that the erg is an exceedingly small unit. For this reason it is customary to employ a unit which is equal to 10,000,000 ergs. It is called a joule in honor of the great English physicist James Prescott Joule (1818-1889). The work done in lifting a liter of water 1 m. is therefore 9.8 joules.

=

=

SUMMARY. Work force times distance; that is, W = F x s. Gravitational units of work are the gram centimeter (g. cm.), the gram meter (g. m.), the kilogram meter (kg. m.), the foot pound (ft. lb.), and the foot ton (ft. t.).

Absolute units of work are the dyne centimeter, or erg, and the joule (= 10,000,000 ergs).