84 Mr. Daniell on the Barometer. 



maintain that my experiment presented absolute proof that the 

 air had insinuated itself between the mercury and the tube, and 

 shewed that there was no " reason to believe that this air existed 

 in mercury in the same invisible state as in water, that is, distri- 

 buted through its pores." For, if the latter had been the case, 

 the mercury, which contained no air after being boiled, would, 

 from its greater density, have sunk in the tube, when surrounded 

 by mercury which had not been boiled, and would have risen 

 gradually as it became saturated with air. I am justified in draw- 

 ing the conclusion from the contrary effect, that the air had insi- 

 nuated itself between the metal and the tube, for the capillary 

 depression is known to be in inverse proportion to the affinity 

 of the fluid for the containing tube, and nothing could have af- 

 fected that affinity in the case before us, but the gradual interposi- 

 tion of a thin stratum of air and moisture. 



Having thus traced and measured the progress of the air down 

 the sides of small tubes filled with mercury, and boiled with the 

 greatest care, I was naturally led to suspect that the same action 

 might take place in barometers, to their gradual deterioration. I 

 soon saw reason to conclude that such a process actually was go- 

 ing on in the most carefully constructed instruments, and that, in 

 time, air would thus insinuate itself into the best Torricellian va- 

 cuum. In the paper upon the construction of the barometer, to 

 which I have before alluded, I gave all the particulars of the 

 making of two barometers, in which every precaution was used 

 to dispel every particle of air. One of these was of very large 

 dimensions, and was fixed up in the apartments of the Royal So- 

 ciety, under the superintendence of the Meteorological Committee. 

 The other was of the mountain construction, and intended for my 

 own use. The agreement between these two instruments, when 

 all corrections were made for the differences in their sizes and 

 forms, was very perfect, and proved that the care which had been 

 bestowed upon them had not been thrown away. In the latter, 

 however, I lately remarked that a small quantity of air had as- 

 cended into the vacuum, I could not discover any way in which 

 this could have obtained admission ; but, attributing it to acci- 



