226 THE POPULAR SCIENCE MONTHLY. 



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 Professor Thomas Andrews, of Belfast, has 

 named it, is continually transferred to the surface by means of convec- 

 tion-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 

 tenrperature of a room is, generally speaking, higher toward the ceil- 

 ing than toward the floor, and that upon plunging a thermometer into 

 a tank of heated water the surface temperature is found slightly supe- 

 rior to that near the bottom. 



These convection-currents owe their existence to a preponderance 

 of the cooled descending over the ascending current ; but this differ- 

 ence being slight, and the ascending and descending currents inter- 

 mixing freely, they are, generally speaking, of a sluggish character ; 

 hence, in all heating apparatus, it is found essential to resort either to 

 artificial propulsion, or to separating walls between the ascending and 

 the descending currents, in order to give effect to the convective 

 transfer of heat. 



In the case of a fluid sun another difficulty presents itself through 

 the circumstance that the vast liquid interior is enveloped in a gaseous 

 atmosphere, which, although perhaps some thousands of miles in depth, 

 represents a relatively very small store of heat. Convection-currents 

 may be supposed active in both the gaseous atmosphere and in the 

 fluid ocean below, but the surface of this fluid must necessarily con- 

 stitute a barrier between the two convective systems, nor could the 

 convective action of the gaseous atmosphere that is to say, the simple 

 up and down currents caused by surface refrigeration be such as to 

 disturb the liquid surface below to any great extent, because each 

 descending current would have had plenty of time to get intermixed 

 with its neighboring ascending current, and would, therefore, have 

 reached its least intensity on arriving on the liquid surface. 



As regards the liquid, its most favorable condition for heating 

 purposes would be at the critical point, or that at which the slightest 

 diminution of superincumbent pressure would make it flash off into 

 gas ; but considering that, by means of conduction and convection, the 

 liquid matter must have assumed, in the course of ages, a practically 

 uniform temperature to a very considerable depth, it follows that the 

 liquid below the surface, with fluid pressure in addition to that of the 

 superimposed gaseous atmosphere, must be ordinary fluid, the critical 

 condition being essentially confined only to the surface. 



Conditions analogous to those here contemplated are met with in 

 a high-pressure steam-boiler, with its heated water and dense vapor 

 atmosphere. Suppose the fire below such a boiler be withdrawn, and 

 its roof be exposed to active radiation into space, what should we ob- 



