ATMOSPHERIC PRESSURE ON THE LEVEL OF THE MERCU.RY. 33 



evaporates : the space which it had occupied in the pores is filled with successive 

 portions of water from within, in virtue of the attraction of the substance of the 

 pores for water. The volume of the water in the tube diminishes, and thus a 

 vacuum arises, in which the mercury is forced to rise by the atmospheric pressure. 

 The space formerly occupied by the water which has evaporated is now filled with 

 mercury. 



When the mercury has reached a permanent level, the external pressure, which 

 acts on the water in the pores of the bladder (and which tends to displace the parti- 

 cles of water) is obviously equal, before air enters, to the attraction which the 

 substance of the bladder has for the particles of water, and these last to each other. 

 Were the attraction less, air would enter, and the particles of water could not main- 

 tain their position. 



The rise of tUe mercury, or its motion towards the surface of the bladder, that is, 

 towards the point where evaporation is going on, is the result of a difference of 

 atmospheric pressure, determined by the evaporation of the water, or of the liquid 

 which penetrates through the bladder, and by the absorbent power of the bladder 

 for that liquid. 



One chief condition of the efficiency of a bladder, in regard to the rise of a 

 coulmn of liquid, is, that it is kept constantly in contact with the liquid, for without 

 this contact the absorbent power cannot manifest itself. 



By the evaporation a continual efflux of water, in the form of vapour, towards 

 the side on which the air lies, is produced ; and by the capillary action of the blad- 

 der on the other side, water is absorbed and retained with a force which counter- 

 poises 12 or more inches of mercury, according to the thickness of the bladder. 



Now, since the rise of the mercury is an effect of the atmospheric pressure, it is 

 plain, that the height to which the mercury rises, must depend to a certain degree 

 on the state of the barometer.* 



In a tube filled with water, and closed with bladder, the absorbent force of which 

 is equal to the pressure of a column of 12 inches of mercury, the mercury rises by 

 evaporation to the height of 12 inches, as long as a column of 12 inches of mercury 

 can be sustained by the external atmospheric pressure. If this external pressure 

 fall below that limit, the mercury in the evaporation tube falls to the same extent, 

 and if there be water above the mercury, this water separates from the bladder. 



This property of bladder, therefore, would appear unaltered at an elevation 

 at which the barometer should stand at 12 inches ; at a still greater elevation, on 

 the contrary, the liquid would separate from the bladder. 



The external pressure has no influence on the amount of the water evaporating 

 in the pores of the bladder ; that amount depends on the hygrometric state of the 

 surrounding air, and on the temperature.! In a rarified air, 

 (provided it can take up moisture,) evaporation goes on more 

 rapidly than in a denser air ; and hence it is clear, that at 

 certain elevations, the effect of the bladder on the level of the 

 liquid is more quickly produced than at the level of the sea. 

 The amount of water which evaporates is directly propor- 

 tional to the surrounding space, and to the temperature and 

 corresponding tension of the liquid. 



When the tube, Fig. 10, is filled with water to 6, then 

 entirely filled with mercury and inverted in mercury, the Fig. 10. 

 mercury, as we have seen, assumes a fixed level. If we 

 now keep the upper or wide end of the tube, which is closed 

 with bladder, immersed in a vessel of water, Fig. 12, we shall 

 find, after a short time, that the mercury sinks in the narrow 

 tube. If its level has been 12 inches above that of the mer- 

 cury in the vessel, it sinks when the bladder is put into 

 water, 3 or 4 inches for example, and remains stationary at 

 8 or 9 inches, without sinking further for the next 12 hours. 



* Influence of the state of the barometer 



t The pressure of the air does not affect the amount of evaporation. 



5 



