In every part of the animal economy, polar derangements in the electro-vital principle are liable to occur. These derangements are always the real foundation of disease. They may be occasioned by a thousand agencies, which act as the _procuring_ cause of disease; but the _proximate_ and _sustaining_ cause is polar disturbance--derangement of the electro-vital poles. Parts which, in health, are relatively positive, may become negative, and that which should be negative may become positive. Or again, a part, naturally positive to its counterpart, may become _excessively_ so, and that which should be relatively negative may become negative to a _morbid degree_. To correct these polar disturbances and restore the normal polarization, is to _cure the complaint_. This is, under the treatment of most physicians, often accomplished by the use of medicines, and by mechanical or surgical agency. We accomplish it by the proper application of the _poles_ of our electrical apparatus. In cases where there is _virus_ to be destroyed, or _abnormal growths_ to be removed, we also secure the _chemical_ action appropriate to these ends by the proper _selection of our current_. It often happens that _mechanical_ or _surgical action_ is demanded. In many _such_ cases, we do not profess to secure normal polarization and consequent cure by means of electricity alone. Yet, in a large proportion of the cases where mechanical or surgical agency is usually thought to be indispensable, we are able to cure by electric action only, since by it we can exert very considerable mechanical force at will; and can also, in many instances, attain much more happily, by means of electricity, the very ends or the _best_ ends which would be aimed at by skillful surgical operations. PRINCIPLES OF PRACTICE. POLAR ANTAGONISM. _When the conducting cords are of equal length_, as commonly they should be, each of the two poles or electrodes produces a polar effect in the patient directly the opposite of that produced by the other. Also, _at any point_ in either half of the circuit, if it be within the person of the patient, the polar effect produced is the very reverse of what is experienced at the corresponding point in the other half of the circuit. And further; each half of the current produces a polar effect, at every point in the parts of the patient through which it runs, the same in _kind_, though differing in _degree_, as is produced immediately under the pole or electrode with which it is connected; yet an effect antagonistic to that which is produced under the other pole, or at the corresponding point in the other half of the current. IMPORTANCE OF NOTING THE CENTRAL POINT.