predominant. Pasteur's Experimental Study of Microbes. The same difficulty arises with Pasteur as with Darwin; his life-work has already been described often and well. Readers unversed in science have only to turn to the _Vie de Pasteur_, written by his son-in-law, Vallery-Radot, to find a luminous account, giving just so much detail as makes the discoveries intelligible and interesting. If shorter sketches are demanded, they exist. We must therefore above all things be brief, and content ourselves with reminding the reader of facts which, in spite of their recent date, are as well known as anything in the history of science. Chemists will claim Pasteur as one of their number, and we do not dispute the claim. Trained in experimental methods by the chemical laboratory, he devoted his best powers to the study of living things, and, without ceasing to be a chemist, became one of the greatest of biologists. Pasteur's chief work was of course the experimental investigation of living particles which float in the air—what we may call _live dust_. Before his day such particles had been seen, named, and classified; some few had been studied in their action and effects. Most of them are plants of low grade, simplified to the last point for the sake of minuteness, on which their ready dispersal depends. _Yeast._—Van Helmont, early in the seventeenth century, when the microscope had not yet become an instrument of research, attempted to investigate the fermentation of beer, and made acquaintance with the properties of the gas which is evolved, his _gas silvestre_, which was afterwards called fixed air, or carbonic acid. Leeuwenhoek about 1680 examined yeast by his microscopes, and discovered that it is made up of globules which often cohere, and that these globules give off bubbles of gas. Then comes a long interval, during which nothing was done to elucidate the process of fermentation. It was not till 1837 that Caignard-Latour and Schwann, independently of each other, showed that yeast-globules multiply by budding, and are therefore to be set down as living things, probably plants of a simple kind. Twenty years more passed without sensible progress; during this time chemists were striving to prove that the alcohol was produced by contact-action, and that the globules were of no practical importance. By the year 1860 Pasteur was engaged upon the problem. It is well known that he arrived at a firm conviction that living yeast-cells are essential to the production of alcohol. It has since been discovered that the enzyme (unorganised ferment of older writers) secreted by living yeast-cells can change sugar into alcohol after the cells themselves have been destroyed, and that other plants besides yeast-cells secrete the same enzyme when deprived of oxygen.