446 



M. Kupffer, to which also I find it impossible to assent. 

 He alleges that the dew-point obtained directly by Daniell's 

 hygrometer is always lower than the truth ; and he as- 

 cribes this to the bad conducting power of glass, by reason 

 of which the opposite surfaces of the ball containing the 

 thermometer will, while refrigeration is proceeding, have 

 different temperatures, so that when the outer surface has a 

 dew deposited on it, the temperature of the inner surface, 

 and that of the ether in contact with it, are sensibly lower. 

 I do not deny that, theoretically speaking, this must be the 

 case; but I certainly doubt much whether the cause assigned 

 can produce any appreciable effect of the kind attributed to 

 it. On the contrary, according to my experience, the ob- 

 served is almost invariably higher than the true dew-point. 

 Such must inevitably be the case when the ether is 

 poured on too rapidly; for we have thus a local reduction of 

 temperature at the surface of the ether in the ball containing 

 the thermometer, considerably greater than that indicated 

 by the instrument, as it merely shows the mean temperature 

 of the entire column of fluid in which its bulb is immersed. 

 In fact, I have frequently observed, under such circum- 

 stances, a ring of dew to be formed, for example, at 44°, 

 and to disappear subsequently, though the temperature of 

 the inner thermometer was kept steadily at tjiis point, or 

 even carried lower, — showing clearly that partial deposition 

 may take place before the true dew-point is attained. The 

 only mode of avoiding this is to pour on the ether very 

 slowly, so as to produce such a gradual lowering of the 

 included thermometer, that the entire of the ether in which 

 it is immersed shall have, at each instant, a temperature which 

 may be considered uniform throughout. As another cause 

 why the observed dew-point is higher than the true, I may 

 mention the augmentation of the humidity of the air in the 

 vicinity of the instrument, by the pulmonary halitus and 

 cutaneous perspiration of the observer ; a cause which must 



