222 PROCEEDINGS OP THE AMERICAN ACADEMY 



hydrate (Fig. 3), and not until no measurable residue was left by 

 a full liter of gas was its condition regarded as satisfactory. The 

 balloon stood in the calorimeter case, and when the cocks were closed 

 with the precautions already described the temperature of the case 

 and the height of the barometer were observed, and although these 

 values are not needed for determining the tare of the empty balloon, 

 they will be used for determining the density of carbonic acid here- 

 after. The balloon was then disconnected and hung on the balance, 

 and its tare found to be 



W = 11.5907 grams. 



The balloon was then transferred to its metal case, and, having been 

 mounted ;ts shown in Plate III., the determination of the weight of 

 its contents proceeded as just described. 



* Since the volume and density of the potash solution used were closely the 

 same in all the determinations, whether compared before or after the absorp- 

 tion, the value of the correction may be assumed to be the same in all cases, 

 and, as has been shown at page 218, amounts to 0.306 m.g. before absorption 

 and 0.321 m.g. after absorption for every change of one tenth of an inch pres- 

 sure or 1° centigrade temperature from the standard of 300 tenths inch and 

 27° C. or 300° absolute temperature. The correct mode of calculating the 

 correction in any case is to reduce both weights to what they would appear to 

 be at II = 300 and T ° = 300 = 27° C. before taking their difference, using 

 the value 0.30G for the smaller of the two, and 0.321 for the larger. 



290.89 at 25° .8 corresponds in buoyancy to 301.09 at 27° 

 299.83 at 27° " " " 299.83 " 



301.09—300 = 1.09 and 1.09 X 0.306 = 0.33 addative. 

 299.83 — 300 = —0.17 and —0.17 X 0.321 = —0.05 subtractive. 



Then 165.3388 + 0.33 = 165.33913 

 174.0132 — 0.05 = 1 74 01315 



8.67402 as before. 



As regards the sign of the correction, notice that, when reduced to H = 300, 

 the buoyancy becomes less and the apparent weight greater if the previous 

 pressure was above 300, and vice versa. 



