MEMOIR XVIII.] THE CHLOR-HYDROGEN PHOTOMETER. 247 



made in the books of chemistry, that artificial light will 

 not affect it, is wholly erroneous. The feeblest gleams 

 of a taper produce a change. No further proof of this 

 is required than the tables given in this communication, 

 in which the radiant source was an oil-lamp. For speed 

 of action no compound can approach it : a light which 

 perhaps does not endure the millionth part of a second 

 affects it energetically, as will be hereafter shown. 



Proofs of the sensitiveness of the instrument. The fol- 

 lowing illustrations will show that this instrument is 

 promptly affected by rays of the feeblest intensity and 

 of the briefest duration. 



When, on the sentient tube, the image of a flame 

 formed by a convex lens is caused to fall, the liquid in- 

 stantly begins to move over the scale, and continues its 

 motions as long as the exposure is continued. It does 

 not answer to expose the tube to the direct emanations 

 of the lamp without first absorbing the radiant heat, 

 or the calorific effect will mask the true result. By the 

 interposition of a lens this heat is absorbed, and the 

 chemical rays alone act. 



If the photometer be exposed to daylight coming 

 through a window, and the hand or a shade of any kind 

 be passed in front of it, its movement is in an instant ar- 

 rested ; nor can the shade be passed so rapidly that the 

 instrument will fail to give the proper indication. 



The experimenter may further assure himself of the 

 extreme sensitiveness of this mixture by placing the in- 

 strument before a window and endeavoring to remove 

 and replace its screen so quickly that it shall fail to give 

 any indication: he will find that it cannot be done. 



Charge a Leyden-jar, and place the photometer at a 

 little distance from it, keeping the eye steadily fixed on 

 the scale ; discharge the jar, and the rays from the spark 



