A HISTORY OF SCIENCE 



vessel will last half an hour in one of Professor Dewar's 

 best vacuum vessels. Thus in one of these vessels a 

 quantity of liquefied air, for example, can be kept for 

 a considerable time in an atmosphere at ordinary tem- 

 perature, and will only volatilize at the surface, like 

 water under the same conditions, though of course more 

 rapidly ; whereas the same liquid in an ordinary vessel 

 would boil briskly away, like water over a fire. Only, 

 be it remembered, the air in "boiling" is at a tem- 

 perature of about one hundred and eighty degrees be- 

 low zero, so that it would instantly freeze almost any 

 substance placed into it. A portion of alcohol poured 

 on its surface will be changed quickly into a globule of 

 ice, which will rattle about the sides of the vessel like 

 a marble. That is not what one ordinarily thinks of 

 as a "boiling" temperature. 



If the vacuum vessel containing a liquefied gas be 

 kept in a cold medium, and particularly if two vacuum 

 tubes be placed together, so that no exposed surface of 

 liquid remains, a portion of liquefied air, for example, 

 may be kept almost indefinitely. Thus it becomes 

 possible to utilize the liquefied gas for experimental 

 investigation of the properties of matter at low tem- 

 peratures that otherwise would be quite impracticable. 

 Great numbers of such experiments have been per- 

 formed in the past decade or so by all the workers with 

 low temperatures already mentioned, and by various 

 others, including, fittingly enough, the holder of the 

 Rumford professorship of experimental physics at 

 Harvard, Professor Trowbridge. The work of Pro- 

 fessor Dewar has perhaps been the most comprehensive 

 and varied, but the researches of Pictet, Wroblewski, 



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