The student must remember, however, that there is a great difference in the power and effective activity between thought-waves sent forth under different circumstances. Some are sent forth idly, and with no focused power or energy of desire and feeling, and such naturally are weak in effect upon others. Others are sent forth vitalized with strong desire and feeling, and focused with a clear ideal and mental picture, and, consequently, exert a far greater degree of effect upon the minds of others with whom they come in contact. The analogy of the waves of electricity holds good here, for just as the electric power may be strong or weak, as the case may be, so may the mental force be strong or weak under different circumstances, and in different individuals. Vibratory Thought Force The vibratory force of thought-waves persist for some time after their original emanation. Here, also we have analogies on the physical plane, as follows: The heat of a room continues for some time after the fire which originally caused it has ceased to burn. Likewise, the air of a room may manifest the perfume of a flower, or extract, long after the latter has been removed from the room. Again, rays of light persist in existence long after the star manifesting them has been blotted out of existence. In the same way thought-vibrations continue to manifest in a place, large or small though its space may be, long after the original sender has passed from that plane--perhaps even long after he has passed from earth life. Mental Atmospheres A well known American writer on this subject has said concerning this point: "There are many places today filled with the thought-vibrations of minds long since passed out of the body. There are places filled with the strong vibrations of tragedies long since enacted there. Every place has a mental atmosphere of its own, the same arising from the thought-vibrations set in motion by the various persons who have inhabited or occupied them. Every city has its own mental atmosphere which has its effect upon persons moving into them. Some are lively, some dull, some progressive, some old-fogyish, some moral, some immoral--the result of the character of the early settlers and leading spirits, of the place in question. Persons moving into these towns are affected by the mental atmospheres thereof, and either sink to the general level, or else, if strong enough, help to change the mental tone of the place. Sometimes a change in conditions bring a large influx of new people, to a town, and the mental waves of the newcomers tend to bring about a marked change in the local mental atmosphere. These facts have been noticed by many observing people who often have not been familiar with the principles underlying and producing the facts which the observers have so clearly discerned." The Contagion of Thought.