96 MB. W. N. SHAW ON HYGROMETBIC METHODS. 



13. I have now accounted for the whole number of 82 experiments made with the 

 modified apparatus (including those in which dew-point readings were taken) for 

 comparing the saturator and the chemical method at different degrees of humidity, 

 with the exception of three, Nos. 7, 25, and 26. No. 25 gave a difference of '07 

 when the humidity was 70, and simply confirms the general result ; No. 7 gave a 

 difference of + 1'73 when the humidity was 87 ; and No. 26 a difference of '57 

 when the humidity was 86 ; these show a wide divergence, for which I am quite 

 unable to account. There is nothing in my note-book which places those observations 

 on a different footing from the rest ; it is true that they were made at a time when I 

 had not fully appreciated the necessity of keeping the bath at a uniform temperature, 

 but the difficulties did not occur with other observations at corresponding humidities. 

 I can only attribute the large differences to clerical errors in entering the weights. 



14. The reductions of the observations have been made by formula 3 of p. 78, 

 with the aid of the table of p. 79. The value there adopted for the specific gravity 

 of steam referred to air at the same temperature and pressure is '622. The table of 

 vapour pressures which I used at first was REGNAULT'S ; those given in the results are 

 derived from the re-calculation in LANDOLT and BOUNSTEIN'S table. The effect of 

 this change is to diminish the saturation pressure by '02 mm. or '03 mm. in each case, 

 and has consequently given to the Table VII. a preponderance of + differences which 

 did not show itself when REGNAULT'S numbers were taken. As the table now 

 stands, 20 observations give a positive difference and 12 a negative one. If the 

 03 mm. be restored 14 observations will have a positive difference, 15 a negative 

 one, while 3 will give zero difference. Almost precisely the same result would 

 be attained by using the value '624 for the specific gravity of steam instead of 

 622. The one observation on saturated air with the negative difference '18 mm. in 

 the second series of experiments bears out the results of the previous experiments 

 with the original apparatus (5). 



The net result of the whole investigation upon the chemical method would seem to 

 show : 



(1.) That if the specific gravity of saturated aqueous vapour referred to air at the 

 same temperature and pressure be assumed to be '624, the calculation of the pressure 

 from the weight of moisture absorbed gives a value agreeing to within O'l mm. (the 

 mean difference being '07 mm.) with that derived from the temperature of saturation, 

 provided that the air is superheated after passing through the saturator. 



(2.) That there is no reason for assigning the differences in the observations to 

 other than experimental errors. 



(3.) That for saturated air, the result of the chemical method is slightly less than 

 the tabulated saturation pressure. This is shown in the first series of experiments, 

 and agrees with REGNAULT'S result. But I think that the later experiments and the 

 observations with the dew-point instrument which follow clearly show that when 

 saturated air is passed along glass tubes at the same or even a slightly higher 



