262 Dr. Hudson's Method for inferring the Dew-point 



that t" has been accurately ascertained, it would evidently fol- 

 low that the values of V thus found for different temperatures 

 of the hygrometer should be proportional to the numbers 

 I have given in the table (4th column), unless " a given vo- 

 lume of air of different temperatures under 30 pressure has 

 different capacities for heat." Hence this very desirable in- 

 formation may be obtained conjointly with the values of 

 V under that pressure. Also, if the same experiments be 

 repeated under a different atmospheric pressure (say 28), we 

 find the values of V for various temperatures of hygro- 

 meter ; and if variation of pressure (the temperature being 

 the same) makes a difference in the capacity of air* estimated by 

 volume^ we find the amount of that difference at the different 

 temperatures of the hygrometer, and of course obtain another 

 valuable addition to our knowledge. The latter point may also 

 be elucidated in the following manner, viz. by covering the 

 balls of a good differential thermometer with linen, and at 

 known temperatures (when pressure varies) wetting one of 

 the balls with some volatile liquid (spirits of turpentine, for 

 example,) whose evaporation will not cause sufficient cold f to 

 produce a deposition of dew (which might disturb the process 

 by the latent heat of the vapour deposited). We may thus as- 

 certain what effect change of density produces on the capa- 

 city of air estimated by volume, when the other circumstances 

 of the experiment are perfectly similar. — Experiments on the 

 depression of temperature produced by the evaporation of 

 this and other volatile liquids, may (I need hardly add) be 



contained in this paper to my friend Dr. Apjohn two or three days after 

 he communicated his method to the Royal Irish Academy, and I have re- 

 cently had the pleasure to learn from him that he has employed both the 

 methods of experimenting on dry air, and on air of a known dew-point 

 (heated artificially), with the view of testing and establishing the correct- 

 ness of his own formula : this (of course), from the reasons mentioned, I am 

 convinced they cannot do ; but I look forward to his intention of publish- 

 ing his experimental results with much interest, as likely to elucidate the 

 entire subject. 



* I am inclined to think that change of atmospheric pressure will not 

 {cceteris paribus) affect the value of V. The expansions of all gases and va- 

 pours (of whatever density) at a given temperature are the same from equal 

 additions to that temperature, and this expansion or increased repulsive 

 forcefc caused, in each case, by the addition or communication of a certain 

 quantity of heat (as distinguished from temperature). The simplest cause, 

 therefore, that would account for the equality of expansion from equal 

 temperature, would be that equal additions of temperature were also equal 

 additions of heat in each case, or, in other words, that the capacities for 

 heat are equal in equal volumes. I need scarcely say, that I consider ex- 

 periments with sulphuric acid in a receiver (more or less exhausted) as 

 completely fallacious. 



f jEther might be used with advantage if the air were very dry. 



