436 SCIENTIFIC THOUGHT. 



in the gaseous state shows the mechanical energy which 

 we call pressure or expansiveness, the attempt was made 

 to explain the phenomena of expansion, pressure, and 

 temperature of gases by a purely mechanical hypothesis. 

 This answered remarkably well. On the assumption that 

 the particles of a perfect gas possess a rectilinear motion, 

 the experimental formulae of Boyle and Mariotte, of Dal- 

 ton, and of Gay-Lussac, could be theoretically deduced. 

 It also became evident that under this conception the 

 forgotten statement of Avogadro must be correct, accord- 

 ing to which equal volumes of different gases, under equal 

 pressures and at equal temperatures, contain an equal 

 number of freely moving particles. 

 36. And when Clausius showed further that in perfect 



Internal 



energy of gascs Only a portion of the quantities of energy which 

 are measured as motion or as heat can be explained 

 by the assumed rectilinear motion of the particles of 

 gases, and that an internal motion of the particles them- 

 selves must be assumed, the new ideas became still more 

 exactly defined ; they included the conception familiar to 

 chemists of compound atoms or molecules. The smallest 

 individual particles of matter in the free state were them- 

 selves not simple bodies, but systems of still smaller 

 particles ; they were molecules composed of atoms ; the 

 symbols of chemists became descriptive of real physical 

 conditions ; the vague notions of radicles, types, or com- 

 pound atoms began to acquire geometrical and mechanical 

 definiteness. 



Thus the atomic theory, known to the ancients, revived 

 by Dalton in the early years of the century, and em- 

 ployed by chemical philosophers for half a century as a 



molecules. 



