AND ETHER WAVES. 89 



mental solution. Suppose 200 men to be scattered 

 equably throughout the length of Pall Mall. By timely 

 swerving now and then a runner from St. James's Pal- 

 ace to the Athenaeum Club might be able to get through 

 such a crowd without much hindrance. But supposing 

 the men to close up so as to form a dense file crossing 

 Pall Mall from north to south: such a barrier might 

 seriousy impede, or entirely stop, the runner. Instead 

 of a crowd of men, let us imagine a column of molecules 

 under small pressure, thus resembling the sparsely-dis- 

 tributed crowd. Let us suppose the column to shorten, 

 without change in the quantity of matter, until the 

 molecules are so squeezed together as to resemble the 

 closed file across Pall Mall. During these changes of 

 density^jwould the action of the molecules upon a beam 

 of heac passing among them resemble the action of the 

 crowd upon the runner? 



We must answer this question by direct experiment. 

 To form our molecular crowd we place, in the first in- 

 stance, a gas or vapour in a tube 38 inches long, the 

 ends of which are closed with circular windows, air- 

 tight, but formed of a substance which offers little or 

 no obstruction to the calorific waves. Calling the 

 measured value of a heat-beam passing through this 

 tube, when empty, 100, we carefully determine the pro- 

 portionate part of this total absorbed when the molecules 

 are in the tube. We then gather precisely the same 

 number of molecules into a column 10.8 inches long, the 

 one column being thus three and a half times the other. 

 In this case also we determine the quantity of radiant 

 heat absorbed. By the depression of a barometric 

 column, we can easily and exactly measure out the pro- 

 per quantities of the gaseous body. It is obvious that 

 1 mercury inch of vapour, in the long tube, would 

 represent precisely the same amount of matter or in 



