Careful experimenters have shown that the electric current, either direct or alternating, has no direct destructive effect on bacteria. In a liquid medium the organisms may be attracted to or repelled from one or the other pole or may arrange themselves in definite ways between the poles (galvanotaxis), but are not injured. However, electricity through the _secondary_ effects produced, may be used to destroy bacteria. If the passage of the electric current _increases the temperature_ of the medium sufficiently, the bacteria will be killed, or if _injurious chemical substances_ are formed (ozone, chlorine, acids, bases, etc.), the same result will follow (see Ozone, pp. 77 and 157). RADIATIONS. Röntgen or _x_-rays and radium emanations when properly applied to bacteria will destroy them. The practical use of these agents for the direct destruction of bacteria in diseases of man or animals is restricted to those cases where they may be applied directly to the diseased area, since they are just as injurious to the animal cell as they are to the bacteria, and even more so. Their skilful use as _stimuli to the body cells_ to enable them to resist and overcome bacteria and other injurious organisms or cell growths is an entirely different function and will not be considered here. PRESSURE. Hydrostatic pressure up to about 10,000 pounds per square inch is without appreciable effect on bacteria as has been shown by several experimenters and also by finding living bacteria in the ooze dredged from the bottom of the ocean at depths of several miles. Pressures from 10,000 to 100,000 pounds show variable effects. Some bacteria are readily killed and others, even non-spore formers, are only slightly affected. The time factor is important in this connection. The presence of acids, even CO₂, or organic acids, results in the destruction of most non-spore formers. MECHANICAL VIBRATION. Vibrations transmitted to bacteria in a liquid may be injurious to them under certain circumstances. Some of the larger forms like _Bacillus subtilis_ may be completely destroyed by shaking in a rapidly moving shaking machine in a few hours. Bacteria in liquids placed on portions of machinery where only a slight trembling is felt, have been found to be killed after several days. Reinke has shown that the passing of strong sound waves through bacterial growths markedly inhibits their development. CHAPTER VII. CHEMICAL ENVIRONMENT. REACTION OF MEDIUM.