io POPULAR SCIENCE MONTHLY. 



of the other kind were similarly picked out they would form a non- 

 metal called chlorine. Each of these smallest portions of table salt, 

 which if divided are no longer salt, is called a molecule of sodic 

 chloride, and each of the parts into which the molecule is divisible is 

 called an atom, of sodium or of chlorine. In dealing with the dimen- 

 sions of these very small portions of matter an inch or a centimeter is 

 too clumsy a unit. To express the size of an atom in fractions of an 

 inch is worse than stating the diameter of an apple in fractions of a 

 mile. Every one knows what is meant by a millimeter ; it is nearly one 

 twenty-fifth part of an inch. A meter is equal to a thousand milli- 

 meters. Suppose a millimeter divided into a thousand parts. Each 

 of these is called a micron and denoted by the Greek letter . This 

 however is still too large a unit of length for measuring the size of 

 atoms, so we again divide the micron into a thousand parts and call 

 each a micromillimeter or micromil, and denote it by the symbol vp. 

 Lord Kelvin's estimate of the diameter of a molecule is that it lies 

 between one hundredth of a micromil and two micromils, that is be- 

 tween .01 //Aiand 2 p-y-. This is certainly a very wide estimate, but it 

 is the best yet to hand, and for present purposes we may take it that 

 an atom is a small portion of matter of approximately one millionth 

 of a millimeter or one micromil (1 fl/1 ) in diameter. On the same 

 scale the wave length of a ray of yellow light is about 0.6 p. or 600 rifi. 

 that is six hundred times the size of an atom. We know nothing as 

 yet about the relative sizes of different kinds of atoms. In the next 

 place as regards the number of molecules in a given space, various dis- 

 tinguished physicists, Maxwell, Kelvin, Boltzmann, Van der Waals and 

 others, have given estimates for the number of molecules in a cubic 

 centimeter of air at ordinary temperature and pressure, which vary 

 between 10 18 and 10 21 or between a million billion and a thousand 

 million billion. All we can do is to take a rough mean of these 

 different values, and we shall consider that in one cubic centimeter of 

 hydrogen or other gas at 0° C. and 760 mm. or freezing point and 

 ordinary pressure there are about 2 X 10 19 or twenty million million 

 million molecules. To understand what this enormous number means 

 we must realize that if we could pick out all the molecules in one cubic 

 inch of air and place them side by side in a row, small as they are 

 individually, the row would extend nearly twice the distance from the 

 earth to the sun. 



Having provided ourselves with a rough idea of the sizes and 

 numbers of the molecules of any gas we proceed to obtain an idea of 

 their weight or mass. Since 11,162 cubic centimeters of hydrogen 

 gas at 0° C. and 760 mm. weigh one gram, it follows from the 

 above facts that each molecule of hydrogen has a mass of nearly 1/10 23 

 of a gram. To weigh these tiny atoms we must therefore take a 



