If two or more gases are introduced into the same space they ultimately diffuse into each other to form a uniform mixture, notwithstanding that the gases may differ widely in density. This property is of very great importance in considering and discussing technical problems concerned with gases, and are explainable by what is known as the "kinetic theory of gases". The molecules of a gas are not contiguous to each other, but separated more or less widely according to the pressure of the gas. The molecules are considered to be in a state of rapid motion. They will impact on the sides of any vessel in which they are confined and on each other, and this impact effect is what constitutes the phenomenon of "pressure". Heat increases the velocity of movement of the molecules; hence, in a given time more impacts will take place and thus the pressure will increase. In the case of a mixture of two or more gases heat, by increasing the rapidity of movement of the molecules, will increase the chances of collision between them on their being brought into mutual proximity and thus the reactivity of the mixture will be increased if the gases are such that they will react together