580 



H Y G R O M E T R Y. 



Hygrotne- inversely as their specific gravities ; and, therefore, the and, consequently, 

 ^y* actual pressure sustained by the air and the vapour, in 

 ""Y""" the supposed circumstances, is equal to 29.25 .098, 

 or 29-152 inches of mercury; and, lastly, the expand- 



Hygrome- 

 try. 





cd rolume 1770.85 must be multiplied by to 



obtain its augmentation by the diminished pressure, 

 which reduces it to 1822.36 cubic inches. 



To propose an example of an opposite description, 

 let it be required to find the actual volume of air con- 

 -tained in a receiver standing over water at the tempe- 

 rature of 60, when the barometrical pressure is 30.45 

 inches, and the level of the water in the inside of the 

 receiver eight inches above its level on the outside ; 

 supposing also the apparent quantity of air to be 850 

 cubic inches. 



The multiplier in the table for the temperature 60 

 being .98256 the reduced volume, under a pressure of 

 30 inches, is 850 x. 98256 or 835.176 cubic inches; 

 and this result corrected for the pressure becomes 



on AK _ _ 0*50 



835.176 X g() , or 846.061 cubic inches. 

 Additional 44, gy means of the apparatus formerly described, 

 e f^r"Lus S ^ a y Lussac examined the tension of vapour when a 

 sac on the " sma " er quantity of moisture was introduced into the 

 elasticity of cylinder AB, than was sufficient to saturate complete- 

 vapour ly the space previously occupied by the air which it 

 mixed with contained ; and in all cases he found, that the elastic 

 an . force of the vapour, in its attenuated state, was affected 

 state! ky variations of pressure, precisely in the same man- 



ner as permanently elastic fluids, the reduction of bulk 

 which it sustained being always inversely proportional 

 to the pressure. Thus, if N represent the bulk of the 

 air, on introducing a single drop of water, the volume 

 N was gradually enlarged to N' ; and allowing a part 

 of the mercury to flow out, until its surface in AB, 

 and the bent tube TT' was the same, the included air 

 in mixture with the vapour was brought to the same 

 pressure as at the beginning of the experiment. If an 

 additional portion of mercury be now allowed to 

 escape,' the surface of the mercury in the bent tube de- 

 scends below the surface of the mercury in the cylin- 

 der AB ; eo that if the difference of level be repre- 

 sented by fi, the elastic force of the mixture of air and 

 vapour will be p h, the quantity p denoting, as be- 

 fore, the barometrical pressure at the time of the expe- 

 riment. If the change of volume resulting from the 

 change of pressure be now examined, it is found, in all 

 cases, to be the same, as would be obtained with dry 

 air ; so that if N" be the space occupied by the mix- 

 ture in its new state of dilatation, we have, invari- 

 ably, 



N' p h 



But 



it is found by experiment, that 

 N' p h /N 



The elastic 

 force of va- 

 pour la- 



the reduc- 

 tion of vo- 

 lume by 

 mechanical 

 pressure. 



A 1 " p 



45. To determine what change this result implies in 

 the elastic force of the vapour, let /'be the force which 

 the mixture exerted when it occupied the space N', and 

 f the force which it exerts under the volume N"; then 

 since p is the pressure of the atmosphere, the elastic 

 force of the air in the receiver, when; together with 

 the vapour it occupied the space N', must have been 

 p f, is now, on account of the enlargement of vo- 



N' 

 lume, reduced to (p /) T . If to this elastic force 



of the air we add the elastic force of the vapour, we 

 obtain for the elasticity of the mixture f'--(p -/)-T-- 



XT " 



Therefore, 



N' 



N' 



And '/'=/G-)- 



This result, which has been deduced by Biot from the 

 experiments of Gay Lussac, demonstrates what had been 

 formerly stated by Dalton, that the elastic force of va- 

 pour, however attenuated the latter may be, changes in 

 all cases with the volume, precisely in the same manner 

 as that of the gases. Hence it may be concluded, that, 

 so long as vapour retains the aeriform state, the quan- 

 tity of it which can exist in mixture with air, is exactly 

 the same as in a vacuum of equal extent, when the 

 pressure and temperature are the same ; and, therefore, 

 the Table ( 39. ) which expresses the weight in grains 

 of a cubic inch of vapour, from zero to 100 of Fahren 

 heitj may be applied with perfect accuracy to deter- 

 mine the weight of the moisture contained in a cubic 

 inch of air, when the tensions or elasticities of both are 

 the same. The only circumstance necessary for this ap- 

 plication of the Table, so important to the purposes of 

 hygrometry, is some means of ascertaining the elasticity 

 of the vapour in admixture with the air. Mr Dalton 

 has suggested one method of doing this, which is ex- 

 tremely simple, as well as susceptible of the greatest 

 accuracy. 



4>6. The method to which we have alluded, is found- Method of 

 ed on the principle, that if vapour, in an attenuated determining 

 state, (that is, in a state such that the tpace which it oc- * e elasti- 

 cupies is capable of holding an additional portionof mois- Murhi 7 *" 

 ture in the vaporous condition,) be cooled down till it mixture 

 just begin to deposit itself in the form of dew, the vo- with air. 

 lume to which it is then reduced must be completely 

 saturated with moisture ; and consequently the vapour 

 in this reduced state must possess the same elasticity as 

 unmixed vapour at the same temperature. In the case 

 of the atmosphere, we can determine the temperature 

 at which this deposition takes place, by presenting to 

 it a body cooled down continuously from the tempera- 

 ture of the air, until its surface begins to be bedewed 

 with moisture ; and for this purpose, no contrivance 

 seems more convenient than that proposed by Mr Dal- 

 ton, which we shall now briefly describe. 



47. Having taken a cylindrical glass vessel, Mr DaV- 

 ton poured cold water into it, the temperature of whicn 

 was gradually reduced by cooling mixtures when ne- 

 cessary. He then carefully watched, till he observed 

 an incipient deposition of moisture on the surface of 

 the jar; after which he examined the temperature 

 the water, and assumed it ES the temperature at which 

 the moisture in the atmosphere would just be retained 

 in a state of vapour. If a deposition of dew took place 

 immediately, he allowed the jar to stand for some mi- 

 nutes to receive an increase of temperature, wiping it 

 from time to time on the outside with a dry linen 

 towel, till it entirely ceased to exhibit the appearance 

 of moisture on its surface, and thrn examined the tem- 

 perature of the water as before. If due precautions be 

 employed, the temperature, at which the dew is formed 

 on the surface of the jar, may be determined to the 



