AND BY AN ELECTRIC SPARK AT A DISTANCE. 189 



this mixture by placing the instrument before a window, and endeavouring to remove 

 and replace its screen so quickly that it shall fail to give any indication ; he will find 

 that it cannot be done. 



845. Charge a Leyden vial, and place the tithonometer at a little distance from it, 

 keeping the eye steadily fixed on the scale ; discharge the jar, and the rays from the 

 spark will be seen to exert a very powerful effect, the movement taking place and ceas- 

 ing in an instant. 



846. This remarkable experiment not only serves to prove the sensitiveness of the 

 tithonometer, but also brings before us new views of the powers of that extraordinary 

 agent, electricity. That energetic chemical effects can thus be produced at a distance 

 by an electric spark in its momentary passage, effects which are of a totally different 

 kind from the common manifestations of electricity, is thus proved ; these phenomena 

 being distinct from those of induction or molecular movements taking place in the line 

 of discharge, they are of a radiant character, and due to the emission of tithonicity ; 

 and we are led at once to infer that the well-known changes brought about by passing 

 an electric spark through gaseous mixtures, as when oxygen and hydrogen are com- 

 bined into water, or chlorine and hydrogen into muriatic acid, arise from a very differ- 

 ent cause than those condensations and percussions by which they are often explained, 

 a cause far more purely chemical in its kind. If chlorine and hydrogen can be made 

 to unite silently by an electric spark passing outside the vessel which contains them, at 

 a distance of several inches, there is no difficulty in understanding why a similar effect 

 should take place with a violent explosion when the discharge is made through their 

 midst, nor how a great many mixtures may be made to unite under the same treat- 

 ment. A flash of lightning cannot take place, nor an electric spark be discharged, 

 without chemical changes being brought about by the radiant matter emitted. 



847. Proofs of the Exactness of the Indications of the Tithonometer. The foregoing 

 examples may serve to illustrate the extreme sensitiveness of the tithonometer ; I shall 

 next furnish proofs that its indications are exactly proportional to the quantities of light 

 incident on it. 



848. As it is necessary, owing to the variable force of daylight, to resort to artifi- 

 cial means of illumination, it will be found advantageous to employ the following method 

 of obtaining a flame of suitable intensity : 



849. Let A B {fig. 104) be an Argand oil lamp, of which the wick is C. Over the 

 wick, at a distance of half an inch, or thereabout, place a plate of thin sheet copper, 

 three inches in diameter, perforated in its centre with a circular hole of the same di- 

 ameter as the wick, and concentric therewith. This piece of copper is represented at 

 d d ; it should have some contrivance for raising or depressing it through a small space, 

 the proper height being determined by trial. On this plate the glass cylinder, e, an inch 

 and three quarters in diameter, and eight or ten inches long, rests. 



850. When the lamp is lighted, provided the distance between the plate, d d, and the 

 top of the wick is properly adjusted, on putting on the glass cylinder the flame instantly 

 assumes an intense whiteness ; by raising the wick it may be elongated to six inches or 

 more, and becomes exceedingly brilliant. Lamps constructed on these principles may 



