of solid bodies must, therefore, be much denser than those of gases, and exert also a more intense effect on the nerves of our eyes, "Light-conductors" differ, therefore, from "light-radiators" by the lesser density of their waves of light; for which reason they cannot, under ordinary circumstances, form "optical molecules," as I expressed on another occasion. How powerfully the condensation of the waves of light affects the eye is shown by the effect of collecting lenses.
The minimum of density which a body must possess to become light-radiating—that is, to become self-luminous to the eye, or to appear a source of light—is just now not known; but one sees, if this view is correct, the possibility of even vapors or dense gases becoming luminous, as Frankland tried to prove. The results of his experiments might even serve as foundation for the lowest limit of density, if it were not so very difficult, nay, even just now impossible, to make such an experiment in a manner so as to exclude every doubt about the assisting influence of solid bodies.—English Mechanic.
I THINK it is safe to assume, on the one hand, that all, or nearly all, mental discipline is of value; and, on the other, that all, or nearly all, knowledge is of value. It will also be conceded that a life spent in disciplining the mind, while the mind so disciplined is never employed in the direction of utility, is a life wasted. It seems equally true that utility, when so narrowed as to relate only to the outward trappings of the human being, is not real utility.
By mental discipline we mean nothing more than habit of mental action. The disciplined mind does not differ in kind from the undisciplined any more than the strength of a man differs in kind from the strength of a child. It is evident that the best mental discipline must be that which prepares the mind to grasp and direct the facts, realities, and influences, on which human well-being depends.
It is thought that there is study which gives mental power or discipline, while it results in little useful knowledge. By a method of study the mind is to be developed into an intellectual Samson, but a blind Samson; and it is hoped and believed that a mind so trained will do something better than to involve itself and others in a common ruin. That is, it is assumed that a habit of thought which does not lead to useful knowledge in school, is the habit of thought which will lead to useful knowledge in common affairs of life.
The study of a dead language is supposed to give this mental discipline. Such study has almost nothing to do with present realities.