unstable and project off a part of their mass. This projected mass is the alpha particle. What is left of the atom after the explosion is a new substance with chemical properties different from those of the original atom. This new atom is, in general, also unstable and breaks down into something else. This process is repeated over and over again until some stable form of atom is reached. Somewhere in the course of this atomic catastrophe some electrons leave the mass; these are beta rays.

According to this point of view, which is now generally accepted, radium is simply one of the stages in the disintegration of the uranium atom. The atomic weight of uranium is 238.2; that of radium, about 226; that of helium, 4.00. Radium would then beuranium after the latter has lost 3 helium atoms. The further disintegration of radium through four additional transformations has been traced. It has been conjectured that the fifth and final product is lead. If we subtract 8 x 4.00 from 238.2, we obtain 206.2, which is very close to the accepted value for lead, namely, 207.2. In a similar way six successive stages in the disintegration of the thorium atom (atomic weight, 232.4) have been found, but the final product is unknown.

508. Energy stored up in the atoms of the elements. In 1903 the two Frenchmen, Curie and Labord, made an epochmaking discovery. It was that radium is continually evolving heat at the rate of about one hundred calories per gram per hour. More recent measurements have given one hundred eighteen calories. This result was to have been anticipated from the fact that the particles which are continually flying off from the disintegrating radium atoms subject the whole mass to an incessant internal bombardment which would be expected to raise its temperature. This measurement of the exact amount of heat evolved per hour enables us to estimate how much heat energy is evolved in the disintegration of one

gram of radium. It is about two thousand million calories,fully three hundred thousand times as much as is evolved in the combustion of one gram of coal. Furthermore, it is not impossible that similar enormous quantities of energy are locked up in the atoms of all substances, existing there perhaps in the form of the kinetic energy of rotation of the electrons. The most vitally interesting question which the physics of the future has to face is, Is it possible for man to gain control of any such store of subatomic energy and to use it for his own ends? Such a result does not now seem likely or even possible; and yet the transformations which the study of physics has wrought in the world within a hundred years were once just as incredible as this. In view of what physics has done, is doing, and can yet do for the progress of the world, can anyone be insensible either to its value or to its fascination?

QUESTIONS AND PROBLEMS

1. Why is it necessary to use a rectifying crystal or an audion in series with a telephone receiver to detect electric waves?

2. Explain why an electroscope is discharged when a bit of radium is brought near it.

3. The wave length of the shortest X rays is about .00000001 cm. How many times greater is the wave length of green light?

X-RAY SPECTRA, PHOTOGRAPHED BY SIEGBAHN

The photograph illustrates a remarkable discovery made in 1913 in Sir Ernest Rutherford's laboratory by a brilliant
young Englishman, Moseley, who at the age of twenty-eight lost his life in the war. Moseley first studied the relations
between the wave lengths, or frequencies, of the X-radiations emitted by different substances, and found that the X-ray
spectra of all substances were very similar, but that as he went to heavier elements the emitted frequencies progressed
in the definite, systematic way shown in the figure. The progression that he found was arithmetical, beginning with
hydrogen, the lightest, and going by ninety-two equal steps to uranium, the heaviest. His work is a strong indication
that there are but ninety-two elements in nature and that these are all related in some very simple way, all of them
probably being built in some way out of hydrogen

APPENDIX

SUPPLEMENTARY QUESTIONS AND PROBLEMS

CHAPTER I. 1. A new lead pencil is 7 in. long. How many centimeters long is it?

2. From the bed rock upon which the Woolworth Building in New York rests to the top of the tower is 278.3 m. How many feet is it?

3. The wing spread of the NC-4 is 126 ft. How many meters is it? 4. How many kilograms are there in the 16-pound shot?

5. Name three uses made of lead because of its great density, and two uses of cork due to its small density.

6. A flask held 2520 g. of glycerin when filled. What was the capacity of the flask in liters? (See table of densities, p. 9.)

CHAPTER II. 1. A standpipe 100 ft. high is filled with water. Find the pressure at the bottom in pounds per square foot and in pounds per square inch.

2. Deep-sea fish have been caught in nets at a depth of a mile. How many pounds pressure are there to the square inch at this depth? (Specific gravity of sea water = 1.026.)

3. If the pressure at a tap on the first floor reads 80 lb. per square inch, and at a tap two floors above, 68 lb., what is the difference in feet between the levels of the two taps?

4. Find the total force against the gate of a lock if its width is 60 ft. and the depth of the water 20 ft. Will it have to be made stronger if it holds back a lake than if it holds back a small pond?

5. Fig. 477 represents an instrument commonly known as the hydrostatic bellows. If the base C is 20 in. square and the tube is filled with water to a depth of 5 ft. above the top of C, what is the value of the weight which the bellows can support?

FIG. 477

Hydrostatic bellows

6. A hydraulic press having a piston 1 in. in diameter exerts a force of 10,000 lb. when 10 lb. are applied to this piston. What is the diameter of the large piston?