This raising of his thoughts far above the sordid realities he is concerned with into the realms of suggestive theory is typical of Pasteur. His was a true creative mind--poetic {304} in its highest sense. The imagination properly controlled is of as great value to the scientist as to the poet. Pasteur's theories were ever pregnant with truth to be. All his life he kept this question of dissymmetry before his mind and hoped to get back to work at it. But opportunity failed. Other and more practical work was destined to occupy the busy half-century of investigation that followed. Most of Pasteur's work, after this first thrilling discovery and its possible significance, is very well known. His meditations on the distinction between material derived from living and non-living sources led him to investigate certain processes called fermentations--before his time considered merely chemical. It is well known that if a dilute solution of sugar be exposed to the air anywhere in the world it will ferment--that is, certain changes will take place in the liquid, some gas will escape from its surface and alcohol will be formed. There are changes that take place in other organic substances--milk, meat solutions, butter, etc.--that resemble quite closely alcoholic fermentations, though the end-product of the process is not alcohol. Pasteur showed that all these supposed chemical changes are really due to the presence of minute living cells, called ferments. During the growth of these cells they split up the substances contained in the material in which they occur, using parts of them for their nutrition. He proved this very clearly for the lactic acid and butyric acid fermentations. Milk was supposed to become sour and butter rancid because they are unstable organic compounds, liable to change in the presence of the oxygen of the air. These changes were now shown to be due to minute living things that grow in the milk and the butter.