_Bacteria._—Another and even more important chapter in the history of air-wafted organisms was opened by the indefatigable Leeuwenhoek. In 1683 he wrote a letter to the Royal Society which makes mention for the first time of bacteria, which he found upon his own teeth, and described as minute rods; some of them moved with surprising agility. For nearly two hundred years little more was done. A few bacteria were named and classified, and there the matter rested until Schwann proved experimentally that putrefaction is just as much the work of living microscopic organisms as alcoholic fermentation. In 1857 and the following years Pasteur not only confirmed the work of Schwann, which had been received by the majority of chemists with distrust, but went on to show that the lactic, butyric, and ammoniacal fermentations also depend upon the activity of bacteria. The happy thought struck him that they might be studied alive—a possibility which he soon realised in practice, and upon which the new science of bacteriology largely rests. From about the year 1873 he began to occupy himself seriously with contagion, which he suspected to be connected with specific aerial germs. Davaine and others had years before observed in the blood of sheep and cattle which had died of "charbon" (anthrax) minute "bâtonnets" (bacilli). Pasteur's published results induced Davaine to ask whether his "bâtonnets" might not be the cause of "charbon." Again, it was Pasteur's results which induced Lister to make experiments in the field of antiseptic surgery. Pasteur wasted no time upon the curiosities of bacterial life. His first studies on fermentation suggested that specific diseases may be propagated by microscopic germs, and that such cases of spontaneous generation as had hitherto escaped refutation might be explained by the access of live dust. The identification and biological history of the organisms interested him only as a step towards sure methods of controlling, and, if necessary, destroying, them; of mitigating their virulence by inoculation; of rendering animals immune against them; or of stamping out the disease by isolation. All this is happily too well known for repetition here. The story, with its many dramatic incidents, can be read in the pages of Vallery-Radot.