A HISTORY OF SCIENCE 



from the dreamings of the eighteenth-century philoso- 

 phers; the electron of J. J. Thompson shows many 

 points of resemblance to the formless centre of Bos- 

 covich. 



Whatever the exact form of the molecule, its outline 

 is subject to incessant variation ; for nothing in molec- 

 ular science is regarded as more firmly established than 

 that the molecule, under all ordinary circumstances, 

 is in a state of intense but variable vibration. The 

 entire energy of a molecule of gas, for example, is not 

 measured by its momentum, but by this plus its en- 

 ergy of vibration and rotation, due to the collisions al- 

 ready referred to. Clausius has even estimated the 

 relative importance of these two quantities, showing 

 that the translational motion of a molecule of gas ac- 

 counts for only three-fifths of its kinetic energy. The 

 total energy of the molecule (which we call "heat") 

 includes also another factor namely, potential energy, 

 or energy of position, due to the work that has been 

 done on expanding, in overcoming external pressure, 

 and internal attraction between the molecules them- 

 selves. This potential energy (which will be recovered 

 when the gas contracts) is the "latent heat" of Black, 

 which so long puzzled the philosophers. It is latent in 

 the same sense that the energy of a ball thrown into 

 the air is latent at the moment when the ball poises at 

 its greatest height before beginning to fall. 



It thus appears that a variety of motions, real and 

 potential, enter into the production of the condition 

 we term heat. It is, however, chiefly the translational 

 motion which is measurable as temperature ; and this, 

 too, which most obviously determines the physical 



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