112 Mr. S. T. Preston on the Nature of what 



rarefied, which is equivalent to increasing the volume of space 

 occupied by it, the side of any cube (representing the mean 

 distance) will be increased in the ratio of the cube root of the 

 number of times the gas is rarefied. In order to take an ex- 

 treme case, let us suppose the gas to have been rarefied a 

 million times (i. e. to one millionth of its normal density) this 

 corresponding to about l^ of an inch of mercury, a mea- 

 sure that would be imperceptible on an ordinary barometric 

 gauge. After this degree of rarefaction the mean distance of 

 the molecules will have been increased to a hundred times, or 

 V^1,000,000. The mean distance of the molecules, after rare- 

 fying a million times, will therefore be about one seventy- 

 thousandth of an inch, a dimension which, if plotted on a scale, 

 would be invisible to the eye. It follows, therefore, that even 

 with this extreme degree of rarefaction (supposing it could be 

 attained with a good mercurial pump), the molecules of gas 

 are still packed so close that their distance, if marked on a 

 scale, would be invisible to the eye, and still 19 billions 

 (dividing the number originally contained in a cubic centi- 

 metre by 1,000,000) of them are enclosed in a cubic centi- 

 metre. This may possibly not harmonize with the general 

 idea of what is called a ^' vacuum." Indeed it may be fairly 

 questioned whether the inconceivable number of 19 billion 

 separate existences accumulated in the narrow limits of a cubic 

 centimetre of space may not be properly regarded as some- 

 thing the very opposite of a " vacuum." The size of a mole- 

 cule may not be of so much influence. Its presence may be 

 the main thing. The presence of 19 billion separate portions 

 of matter in a cubic centimetre of space may conceivably pro- 

 duce an effect on an electric discharge or a radiometer very 

 different from a vacuum. One reason why the distance of the 

 molecules of gas increases with such extreme slowness on rare- 

 fying is evidently due to the fact that the distance only in- 

 creases as the cube root of the number of times the gas is 

 rarefied. If it were imagined to be possible to carry the 

 rarefaction a million times as far as the above extreme 

 limit, there would still be 19 million molecules in a cubic 

 centimetre. 



4. It may therefore be truly said that even the best pump 

 does not increase appreciably the distance of the molecules 

 of gas, inasmuch as the distance mo^^ed through by the mo- 

 lecules of gas under the action of the pump, if plotted on 

 a scale, would be inappreciable to the eye ; or, in other words, 

 when the most powerful pump has done its work, the molecules 

 of gas are still so close that their distances are too small to be 



