November 10, 1916] 



SCIENCE 



659 



tains 5.4 X10 19 atoms, or 2.7 X 10 19 mol- 

 ecules. Relative to this value, Millikan* 

 says : 



To-day we are counting the number of atoms 

 and molecules in a given mass of matter with as 

 much certainty and precision as we can obtain in 

 counting the inhabitants of a city. No census is 

 correct to more than one or two places in a thou- 

 sand, and there is little probability that the num- 

 ber of molecules in one cubic centimeter of gas 

 under standard conditions differs by more than 

 that amount from 27.09 billion billion. 



One gram of radium in equilibrium with 

 its products emits 1.36 X 10 11 alpha par- 

 ticles or atoms of helium per second, or 

 4.20 X 10 1S per year. It has been shown 

 by experiment that one gram of radium in 

 equilibrium produces about 160 cubic milli- 

 meters of helium per year. From this it is 

 evident that 160 cubic millimeters contain 

 4.20 X 10 1S atoms, and that one cubic centi- 

 meter contains 2.6 X 10 19 atoms or mol- 

 ecules. The helium molecule contains but 

 one atom. 



Furthermore, one gram of radium itself 

 emits 3.4 X 10 10 alpha particles per second. 

 Each atom of radium which emits an alpha 

 particle becomes itself an atom of radium 

 emanation which is a gas. It has been 

 determined that one gram of radium is in 

 equilibrium with 0.6 cubic millimeters of 

 radium emanation. The period of average 

 life of radium emanation is comparatively 

 short, and the fraction which decomposes 

 in one second has been definitely deter- 

 mined. Knowing then the volume which is 

 being decomposed, and hence the volume 

 which is being formed in one second, and 

 knowing the number of atoms produced in 

 one second, it is a simple matter to calcu- 

 late that one cubic centimeter contains 

 2.7 X 10 19 atoms or molecules. There is 

 but one atom in a molecule of radium 

 emanation. 



The enormous number of atoms in a 



* Science, January 24, 1913. 



given quantity of matter may be illustrated 

 in the following manner. If the atoms of 

 hydrogen and oxygen in one cubic inch of 

 water were arranged uniformly % 00 of an 

 inch apart in a single layer, they would 

 cover all the continents of the earth sev- 

 eral hundred times. 



Notwithstanding its extremely small size, 

 the atom is complex in its structure, and 

 is made up of parts exceedingly small in 

 comparison with its own size. The prob- 

 lem which now commands so much atten- 

 tion is the determination of the nature 

 and arrangement of these parts. We have 

 seen that the negative electron is a con- 

 stituent of all atoms, but so far no positively 

 charged particle of similar magnitude has 

 been observed. The alpha particles and 

 the particles of the canal rays are posi- 

 tively charged, but these particles are 

 atomic in their magnitude. Every neutral 

 atom must contain a positive charge or 

 charges equal in magnitude to the sum of 

 the negative charges. 



In 1902 Lord Kelvin 5 suggested that the 

 atom consists of a uniform sphere of posi- 

 tive electrification the size of the atom, 

 throughout which are distributed negative 

 electrons of sufficient number to neutralize 

 the positive charge. In 1904 J. J. Thom- 

 son 6 developed this theory mathematically 

 and showed under what conditions such 

 an atom would be stable, and how various 

 configurations would cause periodicities in 

 the properties of elements as observed in 

 the Periodic System. Thomson has given 

 us the most elaborate discussion of atomic 

 structure which has yet been offered, and 

 has accounted in a remarkable manner for 

 the chemical and other phenomena which 

 the different atoms exhibit. During the 

 last few years, however, some phenomena 

 have been observed which would be difficult 



s Phil. Mag., 3, p. 257, 1902. 

 e Ibid., 7, p. 237, 1904. 



