104:8 Dynamic Theory. 



upon polar currents. ( See page 500). The currents of a molecule of a 

 certain shape will be changed when the atoms of the molecule are re- 

 arranged to give it another shape, although the atoms remain the same 

 in number and kind. This is illustrated by the change in the valency 

 of bodies by change in temperature mentioned on page 1035. The 

 valency depends on the number of bonds, which depends on the polar 

 currents, and finally these depend on the shape of the molecules. 



The apparent differences in property between the different forms of 

 allotropic, isomeric, and polymeric bodies ( see chap. 37 ) are obviously 

 due to differences of molecular forms caused by energy in the shape of 

 heat, light, electricity, &c. These agencies likewise effect changes in 

 bodies that alter their power of absorbing light and heat and cause al- 

 terations in the wave lengths of the light they emit when made in- 

 candescent. 



This is shown in spectrum analysis ( see page 404). When a solid 

 body is heated to incandescence, and its light passed through a narrow 

 slit and a prism, the spectrum it gives will be continuous, the colors 

 running from red into orange, yellow, green, blue, indigo, and violet, 

 fading from one to another without gap or break. In this case it is ob- 

 vious that the body is emitting waves of light of all or most of the 

 lengths in the 44th octave, and this must mean that the particles of the 

 body are then in such a variety of shapes and held together in so many 

 various ways, that the ether of the spaces in and about the particles 

 vibrates in all those different tones. Now as the dispersion or separation 

 of the molecules and atoms becomes great enough by the elevation of 

 the temperature, or the action of electricity to turn the solid body into 

 a vapor, the spectrum changes. It becomes discontinuous, there being 

 only a number of lines across the spectrum, some being of one color 

 and some another, and separated from each other by colorless spaces. 

 This is also the character of the spectra of those bodies which are gases at 

 the ordinary temperature. Between these two sorts of spectra most 

 bodies show an intermediate spectrum composed of bands. The band 

 spectrum is produced by a temperature less intense than that producing 

 the line spectrum. 



The explanation of these differences in the spectra of the same body 

 at different degrees of temperature is that the particles of the body are 

 in different states of aggregation, both as to compactness and form. 

 When a body is cold its aggregation is greatest, all its particles hang- 

 ing together, as in a chunk of pig' iron or piece of platinum. There 

 is even at the ordinary temperature a certain d*egree of separation be- 

 tween the particles, entailing upon them a degree of activity which is 

 constantly being given up as heat, and as constantly renewed by fresh 

 accessions from the temperature outside. The waves of this tempera- 



