THE PARALLEL ROADS OF GLEN ROY. 215 



that heat ? We know that it is consumed in the mo- 

 lecular work of vaporization. In the experiment here 

 arranged, the steam passes from the flask through a 

 tube into a second vessel kept at a low temperature. 

 Here it is condensed, and indeed congealed to ice, the 

 second vessel being plunged in a mixture cold enough 

 to freeze the water. As a result of the process we 

 obtain a mass of ice. That ice has an origin very 

 antithetical to its own character. Though cold, it is 

 the child of heat. If we removed the lamp, there 

 would be no steam, and if there were no steam there 

 would be no ice. The mere cold of the mixture sur- 

 rounding the second vessel would not produce ice. The 

 cold must have the proper material to work upon ; and 

 this material aqueous vapour is, as we here see, the 

 direct product of heat. 



It is now, I suppose, fifteen or sixteen years since I 

 found myself conversing with an illustrious philosopher 

 regarding that glacial epoch which the researches of 

 Agassiz and others had revealed. This profoundly 

 thoughtful man maintained the fixed opinion that, at a 

 certain stage in the history of the solar system, the 

 sun's radiation had suffered diminution, the glacial 

 epoch being a consequence of this solar chill. The 

 celebrated French mathematician Poisson had another 

 theory. Astronomers have shown that the solar system 

 moves through space, and ' the temperature of space ' is 

 a familiar expression with scientific men. It was con- 

 sidered probable by Poisson that our system, during its 

 motion, had traversed portions of space of different 

 temperatures ; and that, during its passage through 

 one of the colder regions of the universe, the glacial 

 epoch occurred. Notions such as these were more or 

 less current everywhere not many years ago, and I 

 therefore thought it worth while to show how incom- 



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