350 A SHORT HISTORY OF SCIENCE 



physics and mathematics by means of the spectroscope, the 

 principle of the conservation of energy, the atomic theory, the 

 kinetic theory of gases, and the study of molecular structure. 

 On the other hand, its relations with the organic world are 

 made more clear through the investigation of the compounds of 

 carbon. 



All other sciences, pure and applied, as well as the industries, 

 profit unexpectedly and almost inconceivably by these nine- 

 teenth century advances in physics and chemistry. The older 

 observational and mathematical astronomy achieves a marvellous 

 triumph in the discovery of a new planet Neptune, as related in 

 Chapter XV, and even this is soon rivalled by the startling 

 achievements of the new physical and chemical astronomy. 



Reserving for the following chapter a sketch of the develop- 

 ment of the natural sciences under the ultimately dominant in- 

 fluence of the theory of evolution, we proceed to outline briefly 

 some of the more notable advances in the physical sciences. 



MODERN PHYSICS. - - Some of the main features in the develop- 

 ment of physics in the nineteenth century have been : the working 

 out of consistent theories of light and radiant heat as wave phe- 

 nomena of a peculiar hypothetical medium called the "ether"; 

 the extensive investigation of electrical and magnetic phenomena 

 and the development of an electromagnetic theory even so far as 

 to include optics ; the working out of a kinetic theory of gases 

 with important relations to chemical as well as physical theory ; 

 the elaboration of general theories of matter, force, and energy, 

 all culminating in the crowning discovery of the great unifying 

 principle of the Conservation of Energy. 



HEAT, THERMOMETRY : CARNOT, RUMFORD. The invention 

 of the thermometer has been traced in Chapter XII. To the 

 nineteenth century belongs the determination of an absolute 

 scale l as distinguished from the arbitrary one previously employed. 



The idea that heat is not a substance but a mode of molecular 

 motion arose in the seventeenth and eighteenth centuries, but was 



1 The absolute scale is based on the indirect determination of a temperature 

 (- 273 Centigrade = - 459 Fahrenheit) at which the internal activity which 

 constitutes heat is supposed to cease. 



