296 THE ADDRESSES, LECTURES, ETC., OF 



be argued against this theory that the heat so produced would be 

 liberated throughout its mass, and would have to be brought to the 

 surface by conduction, aided perhaps by convection ; but we know 

 of no material of sufficient conductivity to transmit anything 

 approaching the amount of heat lost by radiation. 



Chemical action between the constituent parts of the sun has 

 also been suggested ; but here again we are met by the difficulty 

 that the products of such combination would ere this have accu- 

 mulated on the surface, and Avould have formed a barrier against 

 further action. 



These difficulties led Sir William Thomson to the suggestion 

 that the cause of maintenance of solar temperature might be found 

 in the circumstance of meteorites, not falling upon the sun from 

 great distances in space, as had been suggested by Mayer and 

 Waterston, but circulating with an acquired velocity within the 

 planetary distances of the sun, arid he shows that each pound 

 of matter so imported would represent a large number of heat 

 units without disturbing the planetary equilibrium. But in 

 considering more fully the enormous amount of planetary matter 

 that would be required for the maintenance of the solar tempera- 

 ture, Sir William Thomson soon abandoned this hypothesis for 

 that of simple transfer of heat from the interior of a fluid sun to the 

 surface by means of convection currents, which latter hypothesis 

 is at the present time supported by Professor Stokes and other 

 leading physicists. 



This theory has certainly the advantage of accounting for the 

 greatest possible store of heat within the solar mass, because it 

 supposes the latter to consist in the main of a fluid heated to 

 such a temperature that if it were relieved at any point of the 

 confining pressure, it would flash into gas of a vastly inferior, 

 but still of an elevated, temperature. It is supposed that such 

 fluid material, or material in the " critical " condition, as Pro- 

 fessor Thomas Andrews of Belfast has named it, is continually 

 transferred to the surface by means of convection currents, that 

 is to say, by currents forming naturally when a fluid substance 

 is cooled at its upper surface, and sinks down after cooling to 

 make room for ascending material at the comparatively higher 

 temperature. It is owing to such convection currents that the 

 temperature of a room is, generally speaking, higher towards the 



