216 PROCEEDINGS OF THE AMERICAN ACADEMY 



It is obvious from the above experiments that the hygroscopic 

 power of phosphoric anhydride becomes impaired before the powder 

 actually deliquesces, and we liave dwelt ou the point because the ex- 

 perience was of great importance in our investigation by leading us 

 to add the second anhydiide tube to our system, and also to watch 

 carefully the condition of its contents and to replace the tube as soon 

 as the least change was perceptible. We attribute the change to a 

 glazing of the amorphous grains due to the forming of a coating of 

 glacial phosphoric acid, but the effect is one that would not be noticed 

 except under such extreme conditions as existed in our work. We 

 now pass to the system of absorption tubes on the other side of the 

 balloon. 



Absorption Apparatus. 



The apparatus for the absorption of the carbonic acid gas must meet 

 very opposite requisitions. In the first place, it must offer a sufficiently 

 large mass of absorbent to the pure gas which first comes to prevent 

 overheating from the rapid chemical combination. In the next place, 

 it must be capable of i-emoving the last traces of carbonic acid from 

 the air which is afterwards drawn through the tubes. Lastly, it must 

 not be so heavy or so complicated that it cannot be promptly and 

 sharply weighed, and the correction for changes of temperature and 

 pressure between successive weighings accurately estimated. 



After trying in various ways to enlarge and modify the different 

 forms of potash bulbs and spiral tubes in general use, (and in these 

 attempts we had the aid of a skilful glass-blower,) we found that we 

 could reach a more accurate result with a system of separate tubes, 

 in spite of the number of weighings they involved. To absorb the 

 great mass of the carbonic acid we used the simplest form of gas 

 washer, shown in Plate III. at A. The bulb had a capacity of 

 200 c.c, and in this we placed 75 c.c. of a solution of potassic 

 hydrate having Sp. Gr. = 1.443. When empty the bulb weighed 

 only 57 grams, and when filled about 165 grams. Assuming that 

 the potash was fully neutralized, less than one half of this amount 

 would be sufficient to combine with nine grams of carbonic acid ; but 

 to insure prompt action and avoid excessive heating the amount 

 given is required. 



The potash tube was followed by a small U tube, four inches long, 

 with ground caps filled with finely granulated soda lime, and weighing 

 when full about 40 grams. (See B, Plate III.) To this was united a 

 similar U tube containing phosphoric anhydride to hold the aqueous 

 vapor, which is brought over by the gas current in no inconsiderable 



