The relationship between gaseous pressure and molecular collisions is explored. The effects on molecular collisions of varying the number of molecules per unit of volume and of varying the temperature are studied. The experimental study of the relative rates of effusion of hydrogen, oxygen, carbon dioxide and sulfur hexafluoride leads to the quantitative relationship among molecular mass, molecular velocity, and absolute temperature. Mechanical models illustrate the experimental observations. Science Consultant: Professor J. Arthur Campbell, Harvey Mudd College, Claremont, California. Wardâs Volume 10, CHEM Study #4106, 21 1/2 minutes. Black & white, 1989.