# Page:Popular Science Monthly Volume 13.djvu/16

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THE POPULAR SCIENCE MONTHLY.

the magnitudes they represent. I take hydrogen gas for my illustration rather than air, because our atmosphere is a mixture of two gases, oxygen and nitrogen, and therefore its condition is less simple than that of a perfectly homogeneous material like hydrogen. The molecular dimensions of other substances, although varying very greatly in their relative values, are of the same order as these.[1]

Dimensions of Hydrogen Molecules calculated for Temperature of Melting Ice and for the Mean Height of the Barometer at the Sea-Level.

Mean velocity, 6,099 feet a second.

Mean path, 31 ten-millionths of an inch.
Collisions, 17,750 millions each second.
Diameter, 438,000, side by side, measure 1100 of an inch.
Mass, 14 (millions{3}) weigh 11000 of a grain.
Gas-volume, 311 (millions3) fill one cubic inch.

To explain how the values here presented were obtained would be out of place in a popular lecture,[2] but a few words in regard to two or three of the data are required to elucidate the subject of this lecture.

First, then, in regard to the mass or weight of the molecules. So far as their relative values are concerned, chemistry gives us the means of determining the molecular weights with very great accuracy; but when we attempt to estimate their weights in fractions of a grain the smallest of our common standards we cannot expect precision, simply because the magnitudes compared are of such a different order; and the same is true of most of the other absolute dimensions, such as the diameter and volume of the molecules. We only regard the values given in our table as a very rough estimate, but still we have good

1. As some of the readers of this journal may be interested to compare these values, we reproduce the "Table of Molecular Data" from Prof. Clerk Maxwell's lecture on "Molecules," delivered before the British Association at Bradford, and published in Nature, September 25, 1873.

Molecular Magnitudes at Standard Temperature and Pressure, 0°C. and 16 c.m.

 RANK ACCORDING TO ACCURACY OF KNOWLEDGE. Hydrogen. Oxygen. Carbonic Oxide. Carbonic Dioxide. Rank I. Relative mass 1 16 14 22 Velocity in metres per second 1,859 465 497 396 Rank II. Mean path in ten billionths (${\displaystyle \scriptstyle 10^{-10}}$) of a metre. 965 560 482 379 Collisions each second—number of millions 17,750 7,646 9,489 9,790 Rank III. Diameter in hundred hillionths (${\displaystyle \scriptstyle 10^{-11}}$) of a metre 58 76 83 93 Mass in ten million million million millionths (${\displaystyle \scriptstyle 10^{-26}}$) of a gramme; 46 736 644 1,012

Number of molecules in one cubic centimetre of every gas is nineteen million million million on 19 (${\displaystyle \scriptstyle 10^{18}}$).

Two million hydrogen molecules side by side measure a little over one millimetre.

2. See Prof. Maxwell's lecture, loc. cit.; also Appletons' "Cyclopædia," article "Molecules."