This is a very effective way of ensuring that material (e.g. surgical dressings) is completely sterilised, and that even the most resistant bacteria with which it may be contaminated are destroyed. Its use is based upon the fact that water boils when its vapour pressure is equal to the pressure of the surrounding atmosphere. This means that if the pressure inside a closed vessel is increased, the temperature at which water inside the vessel boils will rise above 100 degrees centigrade. By adjusting the pressure, almost any temperature for the boiling of the water will be obtained.