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 direction 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 pronounced tendency to resolve themselves into molecules incomparably 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 communicated 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 acquire greater cohesion, while the mean cohesion of the water continues 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 becomes greater. Numerous applications are made of this property—
