298 " CONCERNING THE EFFECT OF GRAVITY 



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77 



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FIG. 3. 



Suppose we have the space A, B, C. D (Fig. 3) filled with 

 a liquid and its vapour, and attempt to form a temperature 

 gradient between A, B and D, C. Then, if we keep the planes 

 A, B and C, D at two fixed and different temperatures, the 

 same amount of heat will not pass out through the one at lower 

 temperature as passes in through the one at higher temperature 

 for two reasons: 



(1) The liquid itself will circulate and do work because 



every liquid changes its specific volume with tem- 

 perature, consequently if it expands with heat, the 

 heated part will rise to the top and give place to 

 cooler and vice versa if it contracts. 



(2) The vapour will circulate from points of higher vapour 



pressure to those of lower. 



It would then be impossible to obtain a permanent fixed 

 horizontal temperature gradient in a liquid. Perhaps this fact 

 has some bearing on ocean currents and trade-winds, since the 

 tendency is for the water at the poles to be colder than that at 

 the equator. Currents of water and water vapour must result. 



The only possible permanent temperature gradient, then, 

 which can exist in a liquid, that changes its specific volume with 

 temperature, is a vertical one. The direction of this gradient 

 will be from top to bottom or vice versa according as the liquid 

 expands or contracts with heat. 



Suppose such a gradient to exist in a solvent, and let solute 

 be introduced without disturbing the solvent till it is saturated 

 at each point. Then if we neglect the change in specific gravity 

 due to the introduction of solute or suppose it to be less than 

 that due to the temperature gradient, there will be an increase 

 or decrease i n the concentration with height, in the case of a 



