BAKING POWDERS. 103 



minute, and so on until the weight of the tubes remains constant. If the work has 

 been done with <lue precaution, the first tube must have lost just as much as the 

 second has gained. Do not exceed the safe speed thus found. ) 



After M has become cool increase the current to full safe speed and aspirate 

 altogether 3 liters, cpntinuing boiling to the end of aspiration. After the tubes have 

 assumed the temperature of the balance-room open for a moment, weigh, note again 

 thermometer and barometer, and apply correction according to the following 

 formulae: 



(A 2 -A 1 )XTand + (B 2 B^xB 

 in which 



A 1 is the temperature at first weighing in C, 

 A 2 is the temperature at second weighing in C, 

 B 1 is the air pressure at first weighing in mm, 

 B 2 is the air pressure at second weighing in mm, 

 and T and B are constants found as follows: 



G= weight of empty tubes. 

 F= weight of iil lings. 

 The specific gravity of glass =2.7. 

 The specific gravity of filling=2.0. 

 The specific gravity of brass =8.5. 

 Change of .weight of 1 cc air with 1 =0.0000039 gram. 

 Change of weight of 1 cc air with 1 nun presstm>=0.0000015 gram. 

 From above follows: 



Volume of tubes and fillings = -f- 



2.7 z.O 

 f^ i i^ 



Volume of brass weights ~f~ 

 8.5 



and 



G F G+F 

 2.7^2.0 8.5 



representing the differential volume affected by temperature and pressure and teing 

 a constant for the tubes. 

 Now, T = V X 0. 0000039 gram, and 



B = V X 0.0000015 gram. 



Observe that rise of temperature makes the air lighter, consequently the tubes 

 heavier. Therefore the correction must be negative. On the other hand, increased 

 pressure has the opposite effect, making the correction positive. 

 Example: 



G = 80. F = 40, from which follows 

 V =35.5, and 

 T =0.00014 gram, and 

 B =0.00005 gram. 

 Now, if A 1 =25, 

 A 2 = 27, 

 B 1 = 759mm, 

 B 8 a=766mm, 

 then the correction for temperature will be 



0.00028 gram, 

 and for air pressure 



0.00015 gram, 

 making a total <>'' 



0.00043 gram. 



