MODERN SUN CULT — STURMER 195 



virtue of this variation they exliibit a most decided diflference in 

 penetrability and in their action on inanimate objects and on living 

 organisms. 



SOURCE OF ULTRA-VIOLET 



An ordinal"}' wood or coul fire emits lipht and heat rays, but no 

 appreciable ultra-violet radiation. If, however, carbon disulphide 

 is burned in a stream of oxygen, the flame which exhibits intense 

 white light, emits also ultra-violet radiation. So ordinary combus- 

 tion can be the source of these mysterious rays. But carbon disul- 

 phide is a substance both dangerous and malodorous, and its use in 

 lamps for the production of ultra-violet is limited to laboratory 

 experiments. 



ULTRA-VIOLET LAMPS 



The practical ultra-violet lamps are types of electric lamps. This 

 applies to those which are used for strictly scientific purposes as well 

 as to the lamps now so largely sold for family use for the irradiation 

 of the human body. To be sure, the lamps for scientific purposes are 

 designed to furnish a greater intensity of radiation, and one embody- 

 ing a wider range of wave lengths. They are also provided with 

 filters to intercept the light waves, which is not a necessity if the 

 lamp is to be employed for therapeutic purposes, as for such uses 

 the accompanying light waves seem to be desirable and certainly 

 not objectionable. 



Structurally the lamps may be divided into two classes: The ex- 

 posed arc ligiits, and the lamps in which the radiation is produced 

 within a bulb or tube. The principle upon which the majority of 

 ultra-violet lamps are constructed is that an electric spark sent 

 through the vapor of certain metals — iron, nickel, zinc, mercury, 

 etc. — produces this radiation. 



A common form of the arc type has carbon electrodes with metal- 

 lic cores, or cores impregnated with metallic ingredients. But also 

 carbons without metallic ingredients may be used, in which case 

 a very wide arc (abtjut 2 inches) is employed. 



The other kind of lamp (always with a bulb or tube) is known as 

 a mercury vapor lamp. Such a lamp may have electrodes of mercury, 

 or these may be of some other metal, such as tungsten; but in either 

 case the gap between the electrodes is occupied by mercury vapor 

 which the current traverses and causes to glow with a greenish light, 

 the invisible ultra-violet rays being produced with the light waves. 

 The light from such a lamp does not include the longest vi.sible waves, 

 that is, it is devoid of the red and orange; and as a consequence the 

 light is colored, for white is obtained when all the wave-lengths of 

 the red, orange, yellow, green, blue, and violet are produced. The 



