ENDOSMOSE. 219 



thoroughly rinsed. The egg is then placed in water, which 

 is renewed every day two or three times. At first 

 the water, after the egg has remained in it for some 

 time, has a distinct acid taste until the acid which has 

 penetrated into the egg has passed out again; but 

 the white of egg will not pass through the pellicle, 

 while a great quantity of water will gradually pene- 

 trate the membrane ; within two days the egg, which 

 had originally a weight of scarcely 50 gr , will swell 

 considerably and attain a weight of about 80 gr . 



3. Aerostatics and Aerodynamics, or, the Equilibrium 

 and Motion of Gaseous Bodies. 



24. Weight of Air. Loss of Weight in Air. The 

 Balloon. It has been shown in art. 2, that air pos- 

 sesses the general properties which are common to all 

 bodies. Air is, like all other bodies, acted on by 

 gravity, and hence it has weight. But the weight of 

 air is very small : the specific gravity of atmospheric 

 air under ordinary circumstances is about ---J -g- ; of the 

 other gases some are rather heavier, some are even 

 lighter than air. 



If a pig's bladder is weighed in a compressed state 

 that is, when empty and again when distended by 

 having air blown into it, the weight of the bladder will 

 be found the same in both cases. But it would be an 

 error to conclude from this that air has no weight ; for 

 the same result would be obtained by weighing the 

 bladder, while immersed in a vessel of water, first in 

 the compressed state and afterwards filled with water. 



