And one discovery but breaks the path for another, and so on indefinitely. Coming upon a new property, the sensitiveness of silver compounds to light, meant a new means of further discovery, the photographic plate. That plate, responsive to rays which fall without response upon the retina, reveals much to us otherwise unknown and unsuspected. Of old when an observer saw nothing, he thought there was nothing to see. We know better now. Thanks to the sensitive plate we have reason to believe that properties, once deemed exceptional, are really universal. Phosphorescence, for ages familiar in the firefly, in decaying logs and fish, now declares itself excitable in all substances whatever, although usually in but slight measure. The case is typical: the polariscope, the spectroscope, the fluoroscope, the magnetometer, the electroscope, each employing as its core a substance of extraordinary susceptibility, detects that quality in everything brought within its play. Thus from day to day matter is disclosed in new wealths of properties, and therefore in new and corresponding complexities of structure. In ages past mankind was on nodding terms with many things, and had no intimate knowledge of anything. With materials before him richer in array than ever before, and better understood than of old, the inventor asks, What properties do I wish in a particular substance? Then, he proceeds to make, if he can, a dye of unfading permanence, an insulator resistant to high temperatures, an alloy which when subjected to heat or cold remains unaltered in dimensions. He finds materials much more under command than a century ago could have been imagined, as the glass manufacture, the alloying industry, the making of artificial dyes, abundantly prove. Edison’s Warehouse as an Aid. Mr. Edison, for aid in finding just the substance he needs for a new purpose, has at his laboratory in Orange, New Jersey, a large store-room filled with materials of all kinds. He may wish a particularly high degree of elasticity, hardness, abrasive power, or what not; to provide these he has gathered a wide diversity of woods, ivories, fibres, horn, glass, porcelain, metals pure and alloyed, alkalis, acids, oils, varnishes and so on. Take one example from among many which might be given from his shelves; he finds that a sapphire furnishes the best stylus wherewith to cut a channel on a phonographic cylinder. Hard, flinty particles from the air are apt to enter the wax, so as to blunt a cutting edge. Diamonds would be best as channelers, but their cost obliges him to choose sapphires as next best; they are purchasable at reasonable prices and last ten years under ordinary conditions of wear. CHAPTER XII PROPERTIES--_Continued_ Producing more and better light from both gas and electricity . . . The Drummond light . . . The Welsbach mantle . . . Many rivals of carbon filaments and pencils . . . Flaming arcs and tubes of mercury vapor. Light Giving Properties.