AMERICAN INSTITUTE. 681 



is of daily occurrence, a black wall is inevitable. Whereas the 

 glasses should be taken off daily and carefully washed, and when 

 lighted, apply the match to the burner, and open the stop cock 

 with caution, permitting no more gas to escape than is sufficient 

 to form a distinct ring of blue flame, then place the glasses on, 

 and raise the flame to the desired height, about three inches. By 

 paying attention to this simple rule, you will never be compelled 

 to complain of broken chimneys or blackened ceilings. 



Many substances have been used for obtaining gas, such as tar, 

 rosin, fat, oil, coal, &c. ; rosin and oil afford a more brilliant and 

 whiter light than coal, because they contain a larger percentage 

 of carbon, and their flame is perfectly free from sulphuretted hy- 

 drogen, and nearly resembles the light of wax candles, but coal 

 being far cheaper is generally used. If coal gas is forced through 

 pumice stone saturated with naptha, it gives a finer light than 

 coal gas alone, and saves eighteen per cent. 



Coal gas often turns white oil paint black, owing to the large 

 quantity of sulphuretted hydrogen contained in it. This may be 

 prevented by covering the paint with varnish. 



What is light? Rays of exceedingly minute particles, radiating 

 in all directions from luminous bodies in straight lines, and when 

 they strike upon solid bodies they rebound. A beam of sun 

 light, though apparently colorless, consists of seven colored rays, 

 forming what are termed prismatic colors ; but these do not pro- 

 duce heat. Rays of heat and light from the sun pass readily 

 through glass, but with fire, the light only, and not the heat, pass 

 through. 



Flame is volatilized matter converted into vapor, and by heat 

 rendered luminous. The flame of a lamp, or candle, is not solid 

 but a thin white vapor, surrounding a heated vapor, which for 

 the want of oxygen is incapable of igniting -, you may insert a 

 tube into this dark centre and ignite the vapor passing through 

 it at the opposite end. When the flame of a candle tapers to a 

 point, there is no smoke, because the carbon is burned at the top- 

 but the moment it flickers, and presents a red appearance at the 

 top, the unburned carbon escapes through the flame, and gives 

 out an abundance of smoke. Argand has remedied this defect by 



