In the world there is nothing invariable. If to conclude under pressure some volume of gas in a tight cover, then certain processes will proceed there. Pressure of gas and its volume will change.
How to hold gas?
The simplest at first sight experience is to inflate a balloon with the most usual air. It is worth taking that circumstance that the ball is made of thin rubber into account, but to give it due volume, air has to be in it under pressure surpassing effort of stretching of rubber.
It is clear, that the rubber layer will be thicker and stronger, the bigger pressure will be required. The camera of an automobile wheel takes the habitual form and necessary elasticity only with an excessive pressure not less than one Bar or the technical atmosphere.
Certainly, excessive pressure in a balloon is much lower. But anyway it is. Thus, the prerequisite for leak of air through the slightest thinnesses is created. The most weak spot in this system is a channel via which the ball is inflated. Rubber is thicker there, thus, when setting a ball the micro channels exceeding the size of a molecule of the gas which is contained in a sphere are formed. Considering elevated pressure inside, the mechanism of leak of air from a ball is quite clear. Continuously squeezed volume of air seeks to come to light through microscopic openings. There are also other reasons that the ball is blown off. Thin rubber rather often has a microscopic time through which air also leaves a cover. It is clear, that can lead substantial increase of pressure to the accelerated air outlet. For this purpose it is quite enough to leave a sphere in the sun. Sunshine will very quickly lift temperature of the air which is contained in a ball and by that will increase pressure in it. Naturally, air will begin to leave a cover much quicker.
Ball with helium
Practically the same situation develops if a sphere to fill with helium. Helium is much lighter than air - and at the expense of this difference the carrying power is provided. That is, if to release the ball filled with helium - it quickly enough will depart up. But the strangest that sooner or later the ball will return! And the cause is there will be a loss of carrying power. The reasons for its reduction a little, and the most obvious are precipitation. The drops of water vapor settling on a surface of the ball filled with helium at some point will exceed indicators of carrying power - and the sphere will direct to the earth. But there is more to come, having dried, the ball will begin to rise again. The sunshine warming its surface will evaporate moisture, but at the same time and will increase pressure of gas and, thereby, will accelerate its leak according to the same scheme, as with air. Even quicker as the size of a molecule of helium is much less than time of a rubber cover.