THE STORY OF NINETEENTH-CENTURY SCIENCE 



ceivable that a stage might be reached at which the 

 molecules became absolutely quiescent, as regards either 

 translational or vibratory motion. Such a heatless con- 

 dition has been approached, but as yet not quite attained, 

 in laboratory experiments. It is called the absolute 

 zero of temperature, and is estimated to be equivalent 

 to 273 Centigrade below the freezing-point of water, or 

 ordinary zero. 



A temperature (or absence of temperature) closely 

 approximating this is believed to obtain in the ethereal 

 ocean of interplanetary and interstellar space, which 

 transmits, but is thought not to absorb, radiant energy. 

 We here on the earth's surface are protected from ex- 

 posure to this cold, which would deprive every organic 

 thing of life almost instantaneously, solely by the thin 

 blanket of atmosphere with which the globe is coated. 

 It would seem as if this atmosphere, exposed to such a 

 temperature at its surface, must there be incessantly 

 liquefied, and thus fall back like rain to be dissolved 

 into gas again while it still is many miles above the 

 earth's surface. This may be the reason why its scurry- 

 ing molecules have not long ago wandered off into space, 

 and left the world without protection. 



But whether or not such liquefaction of the air now 

 occurs in our outer atmosphere, there can be no question 

 as to what must occur in its entire depth were we per- 

 manently shut off from the heating influence of the sun, 

 as the astronomers threaten that we may be in a future 

 age. Each molecule, not alone of the atmosphere, but of 

 the entire earth's substance, is kept aquiver by the energy 

 which it receives, or has received, directly or indirectly, 

 from the sun. Left to itself, each molecule would wear 

 out its energy and fritter it off into the space about it, 



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