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 

 Palace 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 seriously impede, or entirely stop, the 

 runner. Instead of a crowd of men, let us imagine a 

 column of molecules under small pressure, thus resem- 

 bling the sparsely-distributed 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, would the action of the mole- 

 cules upon a beam of heat passing among them re- 

 semble 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 



