The bodies of the planets are in like manner invisible; we behold but the illumined atmosphere of each sphere. Thus the moon and planets, to be visible, must possess atmospheres. _Light as a Substance._ That the thunderbolt is a substance may not be questioned. That the aurora borealis, or polaris, another form of vito-magnetic fluid, is a substance is not questioned. The so-called heat-lightning, though apparently intangible, must therefore be regarded as a substance. Yet further in the remove we find the zodiacal light. Sunlight is but the same, in form of extreme tenuity. The thunderbolt passes from earth to cloud, and instantaneously changes its _substantial_ form to one as tenuous as light; yet, in the transformation, this fluid has not lost its identity. Though unseen, it continues to exist as matter. _Velocity of Light._ While ever present, light is being incessantly replenished; its action being instantaneous. The calculations of ROEMER, founded upon observations made through spaces of 382 and 568 millions of miles of distance, should not be too confidently accepted, especially as the results of such conclusions are so vitally important. When we consider that with our best telescopes directed towards the moon, less than a quarter of a million of miles distant, nothing really satisfactory may be discerned, what value, therefore, may be attached to statements founded upon such thoroughly unreliable data? BRADLEY'S estimate of the velocity of light, founded upon his study of "the aberration of light," is even less worthy of consideration. Any effort to measure such an inconceivable velocity as that claimed for light, by any means or appliances which may be devised by human ingenuity, must be regarded as futile. DESCARTES says: "Light reaches us _instantaneously_ from the sun, and would do so, even if the intervening distance were greater than that between the earth and heaven." FOOTNOTES: [5] Appendix, p. 99. [B] This term is employed as being most exact and comprehensive, as this fluid is now known to be the source of all life and all attractions. CHAPTER V. SUN-HEAT. _Its Source and Limits._ Sun-heat is another product of the same retro-action between the sun and earth; consequently it has the same range and the same boundaries as when it is viewed as light. _Tendencies to unsettle in Science._