-----------------------+-----------+---------+----------+--------- RANK ACCORDING TO | Hydrogen. | Oxygen. | Carbonic | Carbonic ACCURACY OF KNOWLEDGE. | | | Oxide. | 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 (10^{-10})| | | | of a metre | 965 | 560 | 482 | 379 Collisions each | | | | second--number of | | | | millions | 17,750 | 7,646 | 9,489 | 9,720 | | | | RANK III. | | | | Diameter in hundred | | | | billionths (10^{-11})| | | | of a metre | 58 | 76 | 83 | 93 Mass in ten million | | | | million million | | | | millionths (10^{-25})| | | | 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 (10^{18}). Two million hydrogen molecules side by side measure a little over one millimetre. _Dimension of Hydrogen Molecules calculated for Temperature of Melting Ice, and for the Mean Height of the Barometer of 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 1/100 of an inch. Mass, 14 (millions^3) weigh 1/1000 of a grain. Gas-volume, 311 (millions^3) fill one cubic inch. To explain how the values here presented were obtained would be out of place in a popular lecture,[B] but a few words in regard to two or three of the data are required to elucidate the subject of this lecture. [B] _See_ Professor Maxwell's lecture, _loc. cit._; also, Appletons' "Cyclopaedia," article "Molecules."