OF ARTS AND SCIENCES. 213 



Summary. 



Tare of empty balloon, 1st value 2.5573 grams. 



" " " 2d " 2.5572 " 



« u 3d " 2.5574 " 



Mean value, 2.5573 " 



Take op the Empty Balloon by Chemical Method. 



The general theory of this method has already been stated. The 

 balloon is filled with carbonic acid gas by displacement, and the tare 

 taken. The gas is then drawn through a series of absorption tubes, 

 as in the process of organic analysis, and the sum of the increased 

 weights of these tubes gives the weight of the total contents of the 

 balloon at the time the tare was taken. The weight of the balloon 

 filled with carbonic acid gas less the weight of the carbonic acid thus 

 determined is obviously the tare of the empty balloon required. In 

 attempting to perfect this apparently simple experimental method we 

 met with unexpected difficulties, arising from several circumstances. 



In the first place, in order to wash out from the balloon the last 

 traces of carbonic acid, it was necessary to draw through the apparatus 

 a very large volume of air, and the system of purifiers and desiccators 

 needed absolutely to free the atmospheric air from the least admixture 

 of carbonic acid or aqueous vapor was found to be far more extensive 

 than we anticipated. In the second place, since the globe held, at 

 the ordinary temperature of the laboratory and at the average pres- 

 sure of the atmosphere, no less than nine grams of gas, the common 

 potash bulbs used in organic analysis were wholly inadequate for our 

 requirements, and we only succeeded after many trials in finding ;i 

 form of apparatus by which so great an amount of carbonic acid 

 could be determined with the necessary accuracy, that is, within a few 

 tenths of a milligram. The arrangement finally adopted is shown in 

 Plate III. The balloon is represented standing in its covered metallic 

 case, in which it is placed the moment it is taken from the balance. 

 On the right of the balloon and below it is a system of tubes tor 

 purifying and drying the atmospheric air which enters from outside 

 the laboratory by the flexible tube on the extreme right. On the 

 left of the globe is a system of tubes for absorbing the carbonic acid. 

 The flexible tube on the left is connected with a Bunsen pump through 



