152 PROCEEDINGS OF THE AMERICAN ACADEMY 



This interpretation is perhaps most readily imparted by a compari- 

 son of the hot convection currents in the two cases. "When the con- 

 vection current is rising vertically, the medium is cooled by expansion 

 until the precipitation temperature is reached, when all the condensible 

 material appears suddenly, save as it is somewhat retarded by the heat 

 liberated in the act. Immediately afterward the particles become rela- 

 tively dark by radiation. In the horizontal currents a very different con- 

 dition of things obtains. Here the medium does not cool dynamically 

 by expansion, but only by radiation ; hence, since the radiation of the 

 solid particles is enormously greater than that of the supporting gas, 

 practically by that of the particles themselves. Thus after the first 

 particle appears, it must remain at its brightest incandescence until all 

 the material of which it is composed is precipitated. From this we 

 see that such a horizontal current must increase gradually in brilliancy 

 to its maximum, and then suddenly diminish, — an exact accordance 

 with the facts as observed. 



John Hopkins University, Baltimore, 

 September, 1880. 



