OF ARTS AND SCIENCES. 93 



128 grains per hour, contained in a spring stand, was placed 

 in line on an adjusting support. Motion of either the instru- 

 ment or candle allowed the line of direction to be found 

 and maintained constant during the experiment. The can- 

 dle was allowed to burn until the projecting wick dropped 

 its light ash away from the candle, when its burning was 

 constant. 



Dr. Hayes alluded to the fact, that the color afforded by 

 the two beams of light was different ; and this was apparent 

 on the page, that from the gas light being nearly white, while 

 a brown tint was given to the page by the flame of the can- 

 dle. He stated that, as the light of flames is due to the 

 ignition of solid matter, the illuminating power of any com- 

 bustible cannot be inferred from its chemical composition, and 

 although, as a general rule, those gases or vapors which de- 

 posit solid, finely divided matter by heat are found to be 

 the best for illumination, yet the introduction of finely di- 

 vided solid matter into flames composed of hydrogen or at- 

 mospheric air will produce luminous effects with the same 

 variations in color. 



In observing the page, as illuminated by the two rectangles 

 of light, the eye soon accustoms itself to judging of the sharp- 

 ness of the outline of the letters, irrespective of the color of 

 the paper, and by retiring backward slightly, the vanishing of 

 the letters on either side is distinctly marked, and the candle 

 can be adjusted to produce equality. The distance of the gas 

 light from the centre of one mirror being constant, the ratio 

 of the light is learned by dividing the square of the gas dis- 

 tance by the square of the candle distance. 



The illuminating power of the gas burned in this city had 

 been the subject of his experiments, from which he obtained 

 the result, that (for the last nine months) the light from one 

 burner is equal to that of about twenty candles. The best 

 solid material for illumination is the sperm candle ; the illumi- 

 nating powers of wax and sperm candles are as twelve to six- 

 teen. 



