In 1870, in a lecture upon haze and dust, Tyndall demonstrated beautifully and in public the presence of countless particles in the air, as well as that these were the agencies operating to produce undesirable changes in organic substances. Both Pasteur and Tyndall, as well as others, showed, as did also Lister, that heat as well as filtration was sufficient to render these particles innocuous. As the result of these and numerous other experiments, by various observers, which there is no time here to recount, it was gradually and irrefutably established that the gases of the air, _per se_, are powerless to cause fermentation or putrefaction in boiled fluids or tissues, or in material germ-free when exposed. It was sufficient, in order to so purify{320} the air, to either previously heat it or filter it through cotton-wool or through fluids inimical to germ-life, while the boiling of organic material or its subjection to the boiling heat of water was sufficient to destroy all germ-activity in it at the time, or, as we say now, to sterilize it. In this way, and even before any minute and systematic study of bacteria,--i.e., before the inauguration of bacteriology as a separate department of scientific study,--it was practically established that the agencies which produce putrefactive changes or fermentation were minute particles which were ever present in almost every substance, and that by heat or something corresponding to filtration it was possible to remove them or destroy their activity.