AND EXTERNAL RESISTANCE. 301 



any difference whatever in the elevation of the 

 mercury. It was found that pure hydrogen 

 gas, which is considered to be 13 times lighter 

 than atmospherical air, was able to raise the 

 mercury from its level in the basin to 29^ 

 inches, equally and as rapidly as either the oxy- 

 gen or atmospherical air, and more especially as 

 the carbonic acid gas, which is the most ponde- 

 rable of the whole; and that the size of the tube, 

 and consequently the quantity of mercury in it 

 to be raised made no difference whatever ; the 

 mercury in the smallest tube did not rise faster 

 or ascend higher than the mercury in the largest. 

 Although I had not the means of pushing 

 the experiment to a greater length, than that of 

 raising 30lb weight of mercury with four pints 

 of hydrogen gas, which do not weigh 2 grains, 

 I have every reason to believe that these 2 grains 

 weight of hydrogen gas would be capable of 

 elevating, and of preserving in a state of sus- 

 pension to the height of 29 inches in an exhaust- 

 ed tube, a quantity of mercury of the greatest 

 magnitude. To those, indeed, who are not 

 disposed to ascribe the elevation of the mer- 

 cury in an exhausted tube, or water in exhaust- 

 ed pumps, to the expansible power of the 

 external air, I would recommend them to as^ 

 certain the fact by the easiest of possible means. 

 It may be proved by simply placing two instru- 



