OF ARTS AND SCIENCES. 227 



Density of Hydrogen. 



We have for the weight of hydrogen held by the balloon the 

 following data. 



B No. 1, weight of hydrogen at 763.10 mm. and 21°.08 = 0.4164 gr. 

 B No. 2, " " 764.92 « " 20°.52 = 0.4178 " 



B No. 3, « " 758.96 " " 22°.75 = 0.4123 " 



nations. Tlie vohime of the counterpoise was adjusted when the balloon was 

 full of air; but although the compensation may be perfect through all ordinary 

 changes of temperature and pressure, it cannot be regarded as absolute. In 

 a laboratory, such changes seldom amount in the aggregate to more than the 

 equivalent of one twentieth of the normal atmospheric pressure ; and although 

 such a variation might not produce a sensible effect on the apparent weight of 

 the small amount of brass used in our weighings, twenty times this effect, which 

 we should have if the brass weiglits were wholly uncompensated, might be an 

 appreciable quantity. In weighing the balloon the largest amount of brass 

 weights used was about 11.5 grams, which at 300 d. in. and 27° C. displace 

 1.6 ni.g. of air. One twentieth of this value would be barely, if at all, percep- 

 tible, but the whole quantity might cause a serious error in our results. 



The tare of the empty balloon as first found by Regnault's method was 

 2.5573 grams ; and we must assume that in the system consisting of balloon 

 counterpoise and weights there may be outstanding a very small uncompen- 

 sated volume, so that the true tare would be represented by 



Wr = 2.5573 ± w. 



We next find the weight of the balloon filled with hydrogen gas, and it is ob- 

 vious that under standard conditions the true weight, or 



W = 2.5573 ± w + 0.4076 - w', 



where w' represents the buoyancy of air on 0.4076 gram of brass. So the true 

 weight of the balloon filled with air must be 



W" = 2.5573 ± w + 5.8594 - w", 



where w" represents the buoyancy of air on 5 8594 grams of brass. In like 

 manner, the true weight of the balloon filled with carbonic acid gas is 



W" = 2.5573 ± w + 8.9564 - w'", 



where w'" is the buoyancy of air on 8.9564 grams of brass. 



Taking 1.176 m.g. as the weight of 1 c.c. of air at 300 d. in. and 27° C, the 

 standard conditions to which the weights have been reduced, we have 



w' = 0.05ra.g., w" = 0.84 m.g., w'" = 1.28 m. g. ; 



and further, 



W — Wr = 4076 — 00005 gram. True weight of hydrogen. 

 W" - Wr = 5.8594 - 0.00084 " " " air. 



W'"-Wr = 8.9564 -0.00128 " " " carbonic acid. 



These corrections should be applied to the weights before calculating the 

 specific gravities, although they only alter the last values one or two units in 



