THE GROWTH OF PHYSICAL IDEAS 93 



Laplace said: "Give me the position and velocity of all the 

 particles at a given moment and I will predict the state of 

 the world at any future moment." The statistical theory of 

 heat, attributed to Ludwig Boltzmann, the electromagnetic 

 theory of light, and the "fluid" theory of electricity tended 

 to confirm this mechanistic viewpoint. 



The nature of heat attracted little attention in ancient 

 times. Fire was one of Aristotle's four elements, and heat 

 was considered an imponderable substance, to which Antoine 

 Lavoisier s^ave the name caloric. That some substances 

 should absorb heat more readily than others ^\ as ascribed to 

 their greater power of attraction and was expressed as their 

 having greater capacity for heat. 



The first scientist to study heat systematically was Joseph 

 Black, a chemist of Glasgow. He observed that when ice 

 melts, it absorbs heat without undergoing any change in 

 temperature; and Black named the heat which disappears 

 in the process latent heat. Black showed that, in the melting 

 of ice, heat was absorbed equivalent to that made available 

 by the cooling of an equal mass of water through 140° Fahren- 

 heit. Black also discovered that heat is used in the evapora- 

 tion of water. It requires, indeed, nearly seven times as much 

 heat to change a pound of water into steam as to melt a pound 

 of ice. 



The discovery that heat was not a substance was made by 

 Benjamin Rumford and Humphry Davy, who showed by ex- 

 periment that heat could be produced by friction. Rumford 

 was engaged in the boring of cannon in the military work- 

 shops of Bavaria and observed the amount of heat produced 

 by the boring tool. He arranged one experiment in which 

 water was boiled by the heat generated in boring the metal. 

 Davy showed that ice could be melted by friction. These 

 experiments were made at the end of the eighteenth century. 

 At the beginning of the nineteenth century John Dalton ad- 

 vanced his atomic theory (see Chapter VI, page 121), and it 

 was realized that matter consisted of molecules and that its 

 properties might be due to the behavior of these molecules. 



