~Effect of heat.~ High temperatures, on the other hand, will destroy any form of life, whether in the vegetative or latent stage. The temperature at which the vitality of the cell is lost is known as the _thermal death point_. This limit is not only dependent upon the nature of the organism, but varies with the time of exposure and the condition in which the heat is applied. In a moist atmosphere the penetrating power of heat is great; consequently cell-death occurs at a lower temperature than in a dry atmosphere. An increase in time of exposure lowers the temperature point at which death occurs. For vegetating forms the thermal death point of most bacteria ranges from 130 deg.-140 deg. F. where the exposure is made for ten minutes which is the standard arbitrarily selected. In the spore stage resistance is greatly increased, some forms being able to withstand steam at 210 deg.-212 deg. F. from one to three hours. If dry heat is employed, 260 deg.-300 deg. F. for an hour is necessary to kill spores. Where steam is confined under pressure, a temperature of 230 deg.-240 deg. F. for 15-20 minutes suffices to kill all spores. ~Drying.~ Spore-bearing bacteria like anthrax withstand drying with impunity; even tuberculous material, although not possessing spores retains its infectious properties for many months. Most of the dairy bacteria do not produce spores, and yet in a dry condition, they retain their vitality unimpaired for considerable periods, if they are not subjected to other detrimental influences. ~Light.~ Bright sunlight exerts on many species a powerful disinfecting action, a few hours being sufficient to destroy all cells that are reached by the sun's rays. Even diffused light has a similar effect, although naturally less marked. The active rays in this disinfecting action are those of the chemical or violet end of the spectrum, and not the heat or red rays.