THE CLASSIFICATION OF THE SCIENCES 521 



the case with Chemistry that the reader will find that I 

 have included Theoretical Chemistry and Spectrum Ajialy- 

 sis under the head of Precise Physical Science. 



Turning to the system of corpuscles, with which we 

 have dealt in Chapter VIII,, we find in them an excellent 

 basis for classifying the Precise Physical Sciences. In the 

 first place we have the particle and groups of particles 

 forming bodies. The division of Physics dealing with 

 the motion of particles or bodies, or of molecules in bulk, 

 is termed Hlolar Physics from the Latin word moles, a 

 mass or bulk. In Molar Physics we deal with the motion 

 which conceptualises the changes of position in bodies at 

 the surface of the Earth, Mechanics ; with the motion 

 which conceptualises the changes in the planetary system, 

 Planetary Theory ; and with the motion by which we 

 describe changes in the configuration of a planet and its 

 satellites, Lunar Theory. 



After the particle we deal with the molecule, and 

 under Molecular Physics treat especially of those pheno- 

 mena which can be conceptualised by the relative motion 

 of molecules. Here we have to consider the Elasticity, 

 Plasticity, Viscosity, and Cohesion of gaseous, fluid, and 

 solid bodies. By aid of the motion of molecules we treat 

 of the phenomena of Sound, the formation of crystals or 

 Crystallography, the Figure of the Earth, the relative 

 motion of the parts of liquids and gases. Hydromechanics, 

 Aeromechanics, and the Theory of the Tides, the theory of 

 the temperature and pressure in gases, or the Kinetic 

 Theory of Gases, etc. 



Passing to a still simpler corpuscle, the atom, we reach 

 Atomic Physics. The motions we attribute to the concept 

 atom form the basis of Theoretical Chemistry, and of the 

 scientific description of those wonderful lines which 

 appear in the light, transmitted or produced by any 

 chemical substance. The Theory of Spectrum Analysis, 

 based on the elementary motions of the atom, is the 

 source of our knowledge of the chemical constitution of 

 the sun and stars, or of all those descriptions of perceptual 

 experience resumed in Solar and Sidereal Physics. 



