676 POPULAR SCIENCE MONTHLY. 



space the instant the external pressure is removed or diminished. 

 Perfect quiet seems to rule in the film enveloping our particles, 

 but this calm is only relative ; and if we supposed the ultimate 

 particles immensely magnified, we should find the conditions very 

 stormy indeed. We have already mentioned the elastic force 

 within the globule which gives it a tendency to expand. The 

 liquid enveloping it is also subject to a law which is illustrated 

 when wet objects dry, and when a cup of water placed on the scale 

 of a balance is found to be losing weight from day to day. The 

 superficial particles of water have a constant tendency to separate 

 from the rest of the mass and go off as invisible particles of vapor 

 so light that they rise in the ambient atmosphere. This passage 

 from the liquid to the vaporous condition goes on gradually, so 

 that the distance between the molecules becomes greater the 

 nearer we approach the free surface. While this takes place in 

 the radial direction, the movement gives rise in the tangential di- 

 rection to contractile forces that act to give the liquid surface the 

 smallest possible extent. 



We may now suppose ourselves witnessing a struggle between 

 rival particles, some of which are continually trying to escape 

 into the globule of air, while others our gaseous particles are 

 all the time striving to penetrate into the water. The spherules 

 escaping into the air have at the same time an extremely pro- 

 nounced tendency to resolve themselves into molecules incompa- 

 rably more tenuous still, and to produce vapor even lighter than 

 the air. As water is a medium of perfect mobility, each detached 

 spherule gives rise to vibratory movements, and these are com- 

 municated to the whole liquid mass. If we turn our attention to 

 the particles of air, we find them making incessant efforts to lodge 

 themselves in the open parts of the line of battle. As soon as one 

 of them has penetrated between two liquid molecules in vibration, 

 these, obedient to their mutual attraction, make it advance still 

 further; and so on till it reaches the midst of the mass. Thus 

 many particles of air one after another penetrate to the deepest 

 parts of the water, where they are strongly compressed and ac- 

 quire greater cohesion, while the mean cohesion of the water con- 

 tinues to diminish ; and as the particles of vapor passing into the 

 air finally saturate it, so no more particles of air can go into the 

 water after it is saturated with gas. 



It follows that the lower the temperature, and, consequently, 

 the stronger the cohesion of the water, the more considerable may 

 be the quantity of dissolved air ; and for this reason, doubtless, 

 the slightest variation of temperature modifies the power of water 

 to absorb air. We can also easily comprehend that the quantity 

 of air dissolved in water increases as the external pressure be- 

 comes greater. Numerous applications are made of this property 



