168 Dr J. A. Fleming [May 30, 



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

 tween '01 fjLfx and 2 /x/x. 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 millimetre or one micromil (1 jx/jl) in diameter. On the same 

 scale the wave-length of a ray of yellow light is about * 6 /x or 600 /x/y 

 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 

 distinguished physicists, Maxwell, Kelvin, Boltzmann, Van der Waals 

 and otbers, have given estimates for the number of molecules in a 

 cubic centimetre of air at ordinary temperature and pressure, which 

 vary between 10^^ and 10^^, 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 centimetre of 

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

 ordinary pressure there are about 2 x 10^^ or twenty million million 

 million molecules. To understand what this enormous number means 

 we must realise 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 centimetres of hydrogen 

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

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

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

 unit of weight equal to one-billionth of one-billionth of a gran j me 

 and then on this scale the hydrogen molecule weighs 10 such units. 

 We may obtain in another way an illustration of the mass, size and 

 number of the molecules of any gas in the following manner : 



First as to size. We can, in a good Whitworth measuring instru- 

 ment, detect a variation in length of a metal bar equal to one millionth 

 of an inch. This short length would be occupied by 25 molecules 

 placed in a row close together. We can in a good microscope see a 

 small object whose diameter is one hundred-thousandth of an inch. 

 In a small box of this size we could pack 16 million molecules close 

 together. The smallest weight which can be weighed on a very good 

 chemical balance is one hundredth of a milligramme. The united 

 weight of one million million million molecules of hydrogen would 

 therefore just be detectable on such a balance. 



Ultra-Atomic Matter. 



Until a few years ago our knowledge of the divisibility of matter 

 may be said to have ended with the chemical unit, the atom. But 

 of late years information has been steadily accumulating which has 



