34 Dr. Masons Description 



compatible with the theory of Dalton, namely, "that the 

 quantity of vapour, contained in a given space, is indepen- 

 dent of the presence or density of any other elastic fluid 

 with which it forms no intimate combination ; or that the 

 maximum quantity of vapour which can exist in a given 

 space is the same, at the same temperature, as it would be, 

 did that space contain nothing else." Nothing can be more 

 obvious, than that steam in vacuo has no concern with ex- 

 ternal pressure ; and it is as well known, that the maxi- 

 mum force or density of steam in air has nothing to do with 

 the density of that air, being the same as in vacuo. 



From Professor Daniell's experiments, the dew-point is 

 not at all affected by the density or rarefaction of the air, 

 consequently, the tension of the existing vapour is the same 

 as in vacuo ; and the refrigeration of the moistened bulb, 

 being entirely regulated by that tension, no barometrical 

 correction will be required ; neither Mr. Dalton nor Pro- 

 fessor Daniell have ever thought of making corrections for 

 the dew-point. 



The careful experiments of Le Roy, Dalton, Gay Lussac, 

 Daniell, and many others, have completely settled this point 

 and placed it for ever beyond a doubt. I have also verified 

 this by experiment, in finding the dew-point by my hygro- 

 meter. 



Since, then, air acts only mechanically over the process 

 of evaporation ; the only difference that atmospheric pres- 

 sure can make, will be in the time required for the moist- 

 ened bulb to cool down to its ultimate limits, evaporation 

 taking place almost instantaneously in vacuo ; while it is 

 impeded in proportion to the density of the air, the rapidity 

 of evaporation, according to Leslie, being inversely propor- 

 tional to the height of the barometer. As the limits to 

 the process of refrigeration are always constant in the mois- 

 tened bulb while in action, a correct observation may at 

 any time be made by mere inspection. 



Fifth objection. That it is almost impossible to take the 

 heat of the air to any degree of nicety without the observa- 

 tion being affected by the power of radiation, and if radiant 

 caloric be allowed to interfere, the conditions of calculation 

 fail. 



This is perfectly true both with respect to this hygro- 



