MR W. N. SHAW ON HTGBOMETRIO MKTH<>|>> 91 



potash tube in No. 11, but neither of these trials showed that the cause was 

 discovered. The next observation, however, furnished an explanation. The tempera- 

 ture of the saturator was above that of the air, and the chemical method gave a result 

 53 mm. below that of the saturator. On inspecting the connecting tubes t found a 

 very fine deposit of dew upon them ; and, as the apparatus was left in connexion with 

 the saturator during the night, I have no doubt that during the cold part of the night 

 the vapour was condensed in the connecting tubes, and that these were not dry when 

 the observations marked 8 to 12 were made, although I took the precaution of 

 running dry air through them, generally for a quarter of an hour, before each experi- 

 ment. Thenceforward the connecting tube was left in communication, by means of 

 the three-way tap, with a calcium chloride bottle over night, and the tubes were dried 

 more carefully before the observations. The next set of four observations, however, 

 showed that the amount of moisture in the drying tubes was still too great, the 

 pressure differences being '11, '20, '20, *40 mm. respectively. I then thought that the 

 dew-point instrument might be producing some effect by leakage or otherwise, and 

 accordingly removed it. The temperature of the bath was reduced by ice to 9'96 C., 

 and the observation showed a difference of '07 mm. only. More ice was added, and 

 the temperature reduced to 7'39 C. : the difference at that observation was 0'09 mm.; 

 with further reductions of temperature, however, to 4'74 C., 4'82 C., and 3'06 C., 

 the drying tubes got too little moisture, the differences being '47 mm., '68 mm., and 

 '81 mm. respectively. 



Then the temperature was allowed to rise gradually, and the difference changed 

 sign, having the values '19 mm. and '18 mm. at 6'55 C. and 10'42 C. respectively. 

 It appeared that the amount of moisture obtained by the chemical method was 

 practically identical with that given by the saturation temperature, when the 

 saturator was not cooled below 8 C. ; below that, when the temperature was falling, 

 the chemical method gave too small a result ; but, as the temperature rose again, too 

 large a result was obtained. The next series of observations, Nos. 31 to 38 

 (Table IV.), shows this order most clearly. Starting with the temperature of the 

 saturator at 1678 C., and gradually cooling it down to 2'94 C., then allowing it to 

 rise to 6'47 C., I got the following results : 



N 2 



