The Propagation of Heat 191 



126. The chief method of propagation of heat 

 through the substance of a fluid is then by means 

 of convection currents while in solids transmission of 

 heat is brought about by conduction. Thus another 

 distinguishing feature as between fluid and solid bodies 

 may be added to those already mentioned in Chapter v. 



Exercise for Student. 



Try to account for the inception of convection currents in such 

 an experiment as No. 69 by the application of the knowledge already 

 gained concerning expansion by heat, density, and fluid pressure. 



127. When treating of the Kinetic Molecular 

 Theory in Chapter v, it was stated that matter con- 

 sisted of small particles, termed molecules, which were 

 separated from one another by varying distances, 

 according to the state of matter concerned. A question 

 naturally arising from this conception of the con- 

 stitution of matter is " What occupies the spaces 

 between the molecules ? " The view generally held 

 at the present day is that a peculiar form of matter, 

 of density exceedingly small when compared with that 

 of any of the common forms, exists in these interspaces, 

 and extends continuously even into those regions which 

 we are accustomed to look upon as empty. This 

 extremely tenuous substance is termed " ether.". (Care 

 must be taken not to confuse this with the well-known 

 liquid bearing the same name.) Ether fills all space 

 and is a prime and omnipresent constituent of all 

 portions of the universe. Our next subject for study 

 will be the means by which heat is caused to pass 

 through this universal medium, the ether of space. 

 That it does so pass is a matter of common knowledge, 

 since heat is received from the sun, a hot body situated 

 over 90,000,000 miles from our Earth, and the sensible 



