MEMOIR XVIII.] THE CHLOR-HYDROGEN PHOTOMETER. 



249 



As it is necessary, owing to the variable force of day- 

 light, to resort to artificial means of illumination, it will 

 be found advantageous to employ the following method 

 of obtaining a flame of suitable intensity. 



Let A B, Fig. 40, be an Argand oil-lamp of which the 



i<r. 40. 



wick is C. Over the wick, at a distance of half an inch 

 or thereabouts, place a plate of thin sheet-copper, three 

 inches in diameter, perforated in its centre with a circu- 

 lar hole of the same diameter 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 deter- 

 mined by trial. On this plate the glass cylinder, 0, an 

 inch and three quarters in diameter and eight or ten 

 inches long, rests. 



When the lamp is lighted, provided the distance be- 

 tween the plate d d and the top of the wick be properly 

 adjusted, on putting on the glass cylinder the flame in- 

 stantly 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 be purchased in the shops. I have, 

 however, contented myself with using a common Argand 

 study -lamp, supporting the perforated plate d d at a 

 proper height by a retort stand. It will be easily un- 

 derstood that the great increase of light arises from the 

 circumstance that the flame is drawn violently through 



