CHANGE IN WEIGHT DURING CHEMICAL REACTION. 
241 
data necessary for estimating, with the aid of certain tables, the total weight of the 
invisible water skin (as it may be appropriately termed) that had formed upon 
the sphere during the time it was suspended in air saturated with aqueous vapour ; 
presumably this skin had- been removed when the final weighings over sulphuric acid 
were effected. The data, together with the final result, are set forth below :— 
Jena glass sphere - 
Diameter = 3 • 0 cm. 
External area = 28 ■ 4 sq. cm. 
Volume = 14'1 c.c. (Taken = 14 c.c.) 
(Apparent weight in damp air) - (apparent weight in dry air) = 0-118 mgr. 
Reduced barometer = 753 mm. = P. 
Mean absolute temperature in balance case = 295°. 
Pressure of water vapour at 295° A = 20 mm. of mercury = p. 
Weight of I.L. of dry air at 753 mm. and 295° A.= 1 • 185 gr. 
P--38p 273 , 
„ moist „ ^ = 1 179 „ 
Therefore 14 c.c. dry air at 753 mm. and 22° C. weigh -016590 gr. 
and 14 „ moist „ „ „ ,, -016506 „ 
Loss of buoyancy for sphere = difference = -000084 gr. 
The increase in the apparent weight of the sphere is due to the joint effects of two 
causes actiug in the same direction— 
(1) a decrease in the density of the air owing to the presence of aqueous 
vapour; and 
(2) the formation of a water skin upon the sphere. 
The weight of the water skin will therefore be equal to 0T18 — ‘084 = ‘034 mgr. 
From this we find the weight of the water skin upon 1 sq. cm. to be equal to 
■034/28’4 = '0012 mgr. ; and this we accept as the maximum value obtainable under 
the conditions of pressure and temperature that existed during the determination.’^ In 
practice, the maximum density for the water skin would very seldom be realised. In 
connection with some of the experiments carried out during the summer of the year 
1910, a hygrometer was set up quite close to the balance case. The mean air tempe¬ 
rature for some days was approximately equal to 19° C., and the mean reading for the 
wet bulb thermometer for the same period was approximately 16°’5 C. ; the truth 
will therefore be closely approached if we assume the mean weight of the water skin 
per 1 sq. cm. of the glass to be equal to ‘001 mgr. instead of '0012 mgr. as found above. 
Accepting the smaller value, we find the respective weights of the water skins that 
Ihmori, experimenting with boiled-out Jena glass, found that the water skin varied within the limits 
-035 and -068 mgr. per 100 sq. cm., or a mean approximately = -0005 mgr. per 1 sq. cm. (Quoted in 
Landolt’s memoir.) 
VOL. CCXII.-A. 2 I 
