90 



MR. W. N. SHAW ON HY/GROMETRIC METHODS. 



9. We have first to discuss the comparison of the results given by the saturator 

 with those of the chemical method for air which is not saturated. 



The apparatus was mounted on November 7 with the REGNAULT dew-point instru- 

 ment in position, and the first four observations were wasted by the washer between 

 the copper cylinder and its lid being cut through and letting the water in. It was 

 put in order again on November 1 7, and gave no trouble afterwards. An observation 

 was taken with the temperature of the water as it stood in the tank, with the 

 following result : 



No. 5. November 17. Vapour pressure, by saturator . . . 



,, chemical method 



Difference 

 Temperature of air, 12'98 C. 



. 10-63 mm. 



' I0 ' 65 

 . +'02 mm. 



Ice was then added, and when it was melted a second observation was taken, as 

 follows : 



No. 6. November 19. Vapour pressure, by saturator 8'54 mm. 



,, ,, chemical method . . 8'59 



Difference . . +'05 mm. 

 Temperature of air, 14'74 C. 



These results seemed to give quite satisfactory agreement between the two indica- 

 tions of the states of the air. Passing over No. 7 for the present, the next five 

 gave results which were increasingly divergent, viz. : 



TABLE III. 



10. I was unable at first to assign any cause for this increasing difference, and 

 thought that in No. 9 it might be due to the air being supersaturated in passing into 

 the saturator, so in the next experiment, No. 10, I passed the air through a tube of 

 ice before it entered the vertical tube a of the saturator (fig. 5). I tried, moreover, 

 whether it was due to the action of the carbonic acid of the air, and interposed a 



