WILLIAM D. HARKINS 



137 



behavior. Since all of the species which constitute an element are given only one 

 place in the periodic system of the chemists, they are called "isotopes" (iso = "sa.me," 

 to pe = "place"). 



The lightest-known atoms are those of hydrogen with a mass of i.66Xio~^-'gm., 

 but the relative or atomic weight is taken as 1.0078 in order that the atomic weight 

 of oxygen may be exactly the whole number 16. 



The mass of a projectile may be determined by the diilculty of deflecting it from 

 its path when moving at any definite speed, since the momentum (mv) is proportional 

 to the mass at a given velocity. Certain negatively charged particles are thus de- 

 flected about two thousand times more easily than hydrogen atoms. These particles 

 are designated as "negative electrons," and their charge 

 is exactly ecjual in magnitude, within the limits of error of 

 the most exact work, to that carried by positively charged 

 hydrogen in the electrolysis of an acid in water. The 

 tracks of negative electrons in air are shown in Figure 2. 

 It may be noted that these tracks are very irregular, while 

 the tracks given by hydrogen or other atoms are straight. 



Thus, this is true of the atom tracks shown in Figure 

 3. The dim track in the right-hand view is that of posi- 

 tive hydrogen, which is the lightest positively charged 

 particle known, so it is considered as the positive elec- 

 tron, often called the "proton." 



All uncharged atoms are built up from equal num- 

 bers of protons and electrons. In the hydrogen atom 

 there is one proton, which contains almost all of the mass 

 of the atom and acts as its central sun or nucleus, and one 

 electron which is supposed, in the Bohr theory, to move 

 in an orbit around the proton somewhat as the moon 

 moves around the earth. 



It is supposed that the atomic weight of an atomic 

 species gives the number of protons in its nucleus and also the total number of elec- 

 trons in the atom. The nucleus of the helium atom consists of four protons and two 

 electrons so that there is a net positive charge of two on the nucleus and there are, 

 therefore, two negative planetary electrons in the outer part of the atom. Helium is 

 the second element in the chemical periodic table, so its atomic number is 2. It may 

 be noted that the atomic number expresses also the positive charge on the nucleus 

 and the number of outer planetary electrons. 



Fig. 3?;. — Shows two views 

 of the synthesis and disinte- 

 gration of atoms given in Fig. 

 3(1. The track marked H rep- 

 resents the track of a positive 

 electron or proton, shown at 

 the left-hand side of the right- 

 hand view of Fig. s(^. The 

 angles are the same as those 

 in the photograph. 



MASS, ENERGY, AND FREQUENCY OF VIBRATION 



Mass and energy may be considered as the same but measured in different units. 

 Thus I gm. of mass is equivalent to i times the square of the velocity of light (c = 3X 

 10" cm. a second, soc^ = 9Xio"'), or 9X10'" ergs. Thus a i-gm. weight (mass = i gm.) 

 is 9X 10^" ergs of energy. 



In 191 5 Harkins and Wilson showed that, according to the special relativity the- 



