298 WHAT IS LIFE 



Pressure, Atmospheric. The pressure of 14.7 

 pounds per square inch exerted by the atmosphere 

 at sea-level. 



Pressure, Osmotic. If a solution is separated from 

 the pure solvent by a semi-permeable membrane, 

 the solvent will diffuse through the membrane into 

 the solution, a process termed osinosis. The volume 

 of the solution will then increase and its level will 

 rise, thus setting up a hydrostatic pressure, termed 

 the osmotic pressure. Chem. Diet. See van't Hoff 

 Factor i. 



Protoplasm. The accepted name for "living mat- 

 ter." It was so named by von Mohl, and called by 

 Huxley "the physical basis of life." 



Quantum. The quantum of action. It is known 

 as Planck's constant or Planck's element of action 

 (symbol h). The sum and substance of the quantum 

 theory is that radiant energy (sunlight and sim lar 

 forms of energy) is emitted and absorbed by a given 

 source only in units. The value of this unit is equal 

 to hv, in which h, Planck's element of action, 

 ( =6.547 XlO-" ergs) is the same for all sources, and 

 V is the frequency of the source, varying with the 

 source. All leading physicists now recognize h to be 

 a universal constant. 



The quantum theory dates from Dec. 14, 1900, 

 when Max Planck, professor of physics in the Uni- 

 versity of Berlin, presented his thesis on energy 

 quanta before the Deutsche Physikalische Gesell- 

 schaft. Planck was led to his conclusion that the 

 emission of energy is a discontinuous process, 

 through his exhaustive study of black-body radiation. 

 Five years later (in 1905) Einstein formulated the 

 theory that light itself consists of "light-quanta," 

 units having the value hv. Einstein evaluated the 

 work necessary to lift an electron out of its bond in 



