6 THE POPULAR SCIENCE MONTHLY. 



the magnitudes tbey represent. I take hydrogen gas for my illustra- 

 tion 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 molecu- 

 lar 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 T i of an inch. 



Mass, 14 (millions 3 ) weigh ^Vo of a grain. 



Gas-volume, 311 (millions 3 ) till 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 Na- 

 ture, September 25, 18Y3. 



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



RANK ACCORDING TO ACCURACY OF KNOWLEDGE. 



Rank I. 



Relative mass 



Velocity in metres per second 



Rank II. 



Mean path in ten billionths f 10 -') of a metre. 

 Collisions each second number of millions . . 



Tydrogen, 



Rank III. 



Diameter in hundred hillionths (, - u ) of a metre [ 



Mass in ten million million million millionths( 10 - 26 ) of a gramme; 



1 

 1,859 



965 

 17,750 



58 

 46 



Oxygen. 



16 

 465 



560 

 7,646 



76 

 736 



Carbonic 

 Oxide. 



14 



497 



482 

 9,489 



83 

 644 



Carbonic 

 Dioxide. 



22 

 396 



379 

 9,720 



93 



1,012 



Number of molecules in one cubic centimetre of every gas is nineteen million mill- 

 ion million on 19 (,' 8 ). 



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



2 See Prof. Maxwell's lecture, loc. cit. ; also Appletons' " Cyclopaedia," article " Mole- 

 cules." 



