586 



HYGROMETRY. 



Application 

 of tin 1 for- 

 mula to 

 Leslie's hy- 

 grometer, 



This expression will accordingly give the weight in 

 grains, of the moisture in a cubic inch of air ; when/ 6 

 the tntire force of vapour for the temperature of the air 

 * ; D, the difference of the temperature between a na- 

 ked thermometer, and one with a moistened bulb co- 

 vered with tissue paper; and ft, the height of the baro- 

 meter, are known. The value off, is obtained by the 

 Table in 31 ; and that of D and /3 by observation. 



64. The manner in which the formula is expressed, 

 renders it capable of being easily reduced to the diffe- 

 rent graduations of the thermometrical scale. Thus if 

 D be expressed in degrees of Leslie's hygrometer, since 

 the interval between the freezing and boiling points in 

 that instrument is divided into 1000 equal parts, if L 

 denote the number of degrees which it indicates, 



and the 



centigrade 



scale; 



Methods 

 employed 

 by" Mr Les- 

 lie to find 

 the quanti- 

 ty ot mois- 

 ture in the 

 air, in 

 terms of 

 the degrees 

 of his hy- 

 grometer. 





10953 - 



447.4-H 

 And in the centigrade scale, 



P'=. 



1 + .0037.5 1 



P' being the weight in grammes of the moisture in a 

 litre of air, and /3 the height of the barometer in metres. 

 65. As the conclusions which Mr Leslie has deduced 

 from his experiments, lead to results differing in some 

 respects from those we have obtained, it may be proper 

 to give a brief account of the mode of investigation he 

 adopted. This is 'the more necessary, as he expressly 

 mentions that two different methods led to the same re- 

 sults. " One of these methods/' to use his own words, 

 " was in a large close room, to bring an hygrometer, 

 conjoined with a thermometer, successively nearer to a 

 stove intensely heated, and to note the simultaneous 

 indications of both instruments ; or to employ two nice 

 thermometers placed beside each other, and having 

 their bulbs covered respectively with dry and with wet 

 cambric. By taking the mean of numerous observa- 

 tions, and interpolating the intermediate quantities, the 

 law of aqueous solution in air was laboriously traced. 

 But the other method of investigation appeared better 

 adapted for the higher temperatures. A thin hollow 

 ball of tin, four inches in diameter, and having a very 

 small neck, was neatly covered with linen, and being 

 filled with water nearly boiling, and a thermometer in- 

 serted, it was hung likewise in a spacious close room, 

 and the rate of its cooling carefully marked. The ex- 

 periment was next repeated, by suspending it to the end 

 of a fine beam, and wetting with a hair pencil the sur- 

 face of linen, till brought in exact equipoise to some 

 given weight in the opposite scale. 1'en .grains being 

 now taken out, the humid ball was allowed to rest 

 against the point of a tapered glass tube, and the inter- 

 val of time, with the corresponding diminution of tem- 

 perature, observed when it rose again to the position of 

 equilibrium. The same operation was successively re- 

 newed ; but as the rapidity of the evaporation declined, 

 five, and afterwards two grains only, were, at each trial, 

 withdrawn from, the scale. From such a series of facts, 

 it was easy to estimate the quantities of moisture which 

 the same air would dissolve at different temperatures, 



and also the corresponding measures of heat expended 

 in the process of solution. By connecting the range of 

 observations," continues Mr Leslie, " it would appear, 

 that air has its dryneas doubled at each rise of temper- 

 ature, answering to 15 centesimal degrees. Thus at 

 the freezing point, air is capable of holding a portion of 

 moisture represented by 1 00 degrees of the hygrome- 

 ter; at the temperature of 15 centigrade, it could con- 

 tain 200 such parts ; at that of 30, it might dissolve 

 400 ; and at 45 in the same scale 800. Or, if we rec- 

 kon by Fahrenheit's divisions, air absolutely humid, 

 holds at the limit of congelation the hundred and six- 

 tieth part of its weight of moisture ; at the tempera- 

 ture of 59 the eightieth part ; at that of 86" the for- 

 tieth part ; at that of 113 the twentieth part ; and at 

 that of 140 the tenth part. While the temperature, 

 therefore, advances uniformly in arithmetical progres- 

 sion, the dissolving power which this communicates to 

 the air mounts with the accelerating rapidity of a geo- 

 metrical series." 



66. Before we examine the results of these experi- 

 ments, which we have no doubt were conducted with 

 every attention to accuracy, we may state, that the 

 conclusion which has been drawn from them respect- 

 ing the law of aqueous solution, is totally irreconcile- 

 able with the results obtained by Dalton and Gay Lus- 

 sac. This will appear, by comparing the weight of a 

 cubic irrch of vapour for the various temperatures, in 

 Table 31, deduced from their experiments; and ac- 

 cording to which, the solving power -follows a different 

 law from that stated by Leslie, though chiefly, we be- 

 lieve, on account of its simplicity, it is the one generally 

 admitted. It will be seen by the following Table, that 

 if the temperatures be taken in an arithmetical progres- 

 sion, the quantities of moisture held in solution, form 

 a succession of quantities, the terms of which increase 

 in a faster ratio than the terms constituting a geome- 

 trical series. 



Hygrome- 

 try. 



Discrep*n 

 cy between 



Mr Leslie's 

 conclusions 

 and the law 

 of aqueous 

 solution de- 

 termined by 

 our investi- 

 gations. 



According to the experiments of Leslie, the solving Hate at 

 power is doubled every 27 degrees ; whereas, accord- which the 

 ing to our Table, this takes place at different inter, solving 

 vals, which increase slowly with the temperature, the P "" in- 

 mean being 234 from the freezing point to 100 of " 

 Fahrenheit. 



67. We shall now compare the result furnished by our Compare 

 formula, with that derived by Mr Leslie from his mode tive resultj, 

 of determining the point of deposition ; and we shall as deduced 

 take the example from his Treatise on the Relations of bv Mr 

 Air to Heat and Moisture. " Suppose," says he, " the .'j 8 

 hygrometer to mark 52, while its wet ball has a tem- mula> 

 perature of 20 centesimal degrees, or 68 by Fahren- 

 heit ; the dissolving power of air at this temperature 

 being 252,* its distance from absolute humidity will 

 therefore be 200, which is the measure of solution an- 

 swering to 1 5 centesimal degrees, or 59 by Fahren- 



* Mr Leilie doe not describe how this number is obtained ; but it is obviously, according to his views, the tenth term of a geometrical 

 teiies, of which the fint term is 200, the last term 400, and number of terms 28. 



