Matter, as known to men in bodies, is seen and felt by means of the physical senses; but to beings not provided with such senses, the things of our world are as invisible and intangible as things of more ethereal substance are to our grosser senses. Elementals which find their habitat in the interior of the earth's shell, usually called _gnomes_, are not conscious of the density of the element of earth as we perceive it; but breathe in a free atmosphere, and behold objects of which we cannot form the remotest conception. In like manner exist the _undines_ in water, _sylphs_ in air, and _salamanders_ in fire. The elementals of the air, sylphs, are said to be friendly towards man; those of the water, undines, are malicious. The salamanders can, but rarely do, associate with man, "on account of the fiery nature of the element they inhabit." The pigmies (gnomes) are friendly; but as they are the guardians of treasure they usually oppose the approach of man, baffling by many mysterious arts the selfish greed of seekers for buried wealth. We, however, read of their alluring miners either by stroke of pick, or hammer, or by floating lights to the best mineral "leads." Paracelsus says of these subterranean elementals that they build houses, vaults, and strange-looking edifices of certain immaterial substances unknown to us. "They have some kind of alabaster, marble, cement, etc., but these substances are as different from ours as the web of a spider is different from our linen." These inhabitants of the elements, or "nature-spirits," may, or may not be, conscious of the existence of man; oftentimes feeling him merely as a force which propels, or arrests them; for by his will and by his thought, he acts upon the astral currents of the aerial world in which they live; and by the use of his hands he sways the material elements of earth, fire, and water wherein they are established. They perceive the soul-essence of man with its "currents and forms," and they also are capable of reading such thoughts as do not spiritually transcend their powers of discernment. They perceive the states of feeling and emotions of men by the "_colors_ and impressions produced in their auras," and may thus irresistibly be drawn into overt action upon man's plane of life. They are the invisible _stone-throwers_ we hear of so frequently, supposed to be _human_ spirits; the perpetrators of mischief, such as destruction of property in the habitations of men, noises, and mysterious nocturnal annoyances.