These few instances do not disclose any general principles which may be applied either for the growth or for the distinction of aërobes or facultative anaërobes. It has been shown that compressed oxygen will kill some bacteria but this method of destroying them has little or no practical value. Oxygen in the form of ozone, O₃, is rapidly destructive to bacteria, and this fact is applied practically in the purification of water supplies for certain cities where the ozone is generated by electricity obtained cheaply from water power. The same is true of oxygen in the “nascent state” as illustrated by the use of hypochlorites for the same purpose. It was stated (p. 74) that certain thermophil bacteria in the soil have an optimum temperature for growth _in the air_ which is much higher than is ever reached in their natural habitat and that they grow at a moderate temperature under _anaërobic_ conditions. It has been shown that if these organisms are grown with aërobes or facultative anaërobes they thrive at ordinary room temperature. These latter organisms by using up the oxygen apparently keep the tension low, and this explains how such organisms grow in the soil.[6] OSMOTIC PRESSURE. Like all living cells bacteria are very susceptible to changes in the density of the surrounding medium. If placed in a medium less concentrated than their own protoplasm water is absorbed and they “swell up”; while if placed in a denser medium, water is given off and they shrink (plasmoptysis or plasmolysis). Should these differences be marked or the transition be sudden, the cell walls may even burst and the organisms be destroyed. If the differences are not too great or if the transition is made gradually, the organisms may not be destroyed, but will either cease to grow and slowly die out, or will show very much retarded growth, or will produce abnormal cell forms. This is illustrated in the laboratory in attempting to grow bacteria on food material which has dried out. A practical application of osmotic effects is in the use of a high percentage of sugar in preserving fruits, etc., and in the salting of meats. Neither the cane-sugar nor the common salt themselves injure the bacteria chemically, but by the high concentration prevent their development. In drying material in order to preserve it there are two factors involved: first, the loss of water necessary for growth and second, the increased osmotic pressure. In a medium of greater density diffusion of water is outward from the cell and this will continue until an equilibrium is established between cell contents and medium. Food for the organism _must be in solution and enter the cell by diffusion_. Therefore, growth ceases in a medium too dense, since water to carry food in solution does not enter the cell. ELECTRICITY.