The discovery, says Silliman, of the highly paramagnetic character of oxygen gas, and of the neutral character of nitrogen, the two constituents of air, is justly esteemed a fact of great importance in studying the phenomena of terrestrial magnetism. We thus see that one-fifth of the air by volume consists of an element of eminent magnetic capacity, after the manner of iron, and liable to great physical changes of density, temperature, &c., and entirely independent of the solid earth. In this medium hang the magnetic needles used as tests, and as this magnetic medium is daily heated and cooled by the sun's rays, its power of [Page 254] transmitting the lines of magnetic force is then affected, influencing undoubtedly the diurnal changes of the magnetic needle. For a complete digest of Faraday's discoveries in diamagnetism the reader is referred to the second edition of Dr. Noad's comprehensive and learned work entitled "A Manual of Electricity." Coming always from the highest walks of philosophy to lower and "_common things_" one cannot help being reminded of the old-fashioned method of _drawing up_ a sluggish fire, and the natural query is suggested whether the poker is to be considered as a weak magnet, and does influence and draw towards the fire a greater supply of magnetic oxygen gas? (Fig. 245.) [Illustration: Fig. 245.] [Illustration: The interior of the optical box at the Polytechnic--looking towards the screen. The assistants are supposed to be showing the dissolving views.] [Page 255] CHAPTER XXI. LIGHT, OPTICS, AND OPTICAL INSTRUMENTS. [Illustration: Fig. 246. "The moon shines bright:--In such a night as this."--_The Merchant of Venice._] "To gild refined gold, to paint the lily, To throw a perfume on the violet, To smooth the ice, or add another hue Unto the rainbow, or with taper light To seek the beauteous eye of heaven to garnish, Is wasteful and ridiculous excess." Perfection admits of no addition, and it is just this feeling that might check the most eloquent speaker or brilliant writer who attempted to offer in appropriate language, the praises due to that first great creation of the Almighty, when the Spirit of God moved upon the face of the waters and said, "Let there be light." If any poet might be permitted to laud and glorify this transcendant gift, it should be the inspired Milton; who having enjoyed the blessing of light, and witnessed the varied and beautiful phenomena that accompany it, could, when afflicted by blindness, speak rapturously of its creation, in those sublime strains beginning with-- [Page 256]