124- Professor Forbes on the Colour of 



gas, through which I from time to time viewed a distant gas 

 flame, and compared it with the colour of the sHt. The ex- 

 periment was performed under 50 and 55 pounds many times 

 over. The light examined was then caused to pass through 

 the steam only 10 inches from the orifice of the stop-cock, 

 under the idea that though the colour there was fainter, pos- 

 sibly there might be a tendency to deveiope lines in the spec- 

 trum. But the experiment being made under the same pressure 

 as before, the effect was similar, only much less intense : the 

 slit had now but a faint tawny colour, and prismatic analysis 

 showed the violet alone absorbed. 



Steam blowing off at 25 pounds, the lantern and slit 20 

 inches above the orifice as at first. To the eye the light ap- 

 peared as red as under 55 pounds. Mr. Edington observed 

 that the colour was deeper than that of the nitrous acid gas 

 bottle. Neither he nor his assistant ever observed the colour 

 of steam before. Prismatic phainomena as before ; only the 

 obscuration not quite so great. 



Steam blowing off at 15 pounds. " Evidently redder than 

 the gas bottle : same phjenomena of spectrum, but green re- 

 mains pure throughout, and verges on (bounds immediately 

 with) orange. During the absorption of violet before vanish- 

 ing (the steam-cock being gradually opened) it assumes a dirty 

 white colour, verging on yellow and purple." A common 

 lamp was viewed through different parts of the column of 

 steam of this pressure, from the orifice up to a height of 5 or 

 6 feet, and wherever it was not entirely obscured, it appeared 

 of different shades of smoke colour up to an intense tawny 

 orange. 



With 7 pounds on the inch. Still visibly red to the eye ; 

 prismatic phgenomena similar but slighter. 



With 4 pounds, no longer visibly red to the eye when ar- 

 ranged as above ; and seen with the prism the violet appears 

 but little affected. When let off in large quantity from the 

 safety-valve, and a lamp viewed through it, there is a faint 

 redness close to the orifice, but everywhere above, the transi- 

 tion is fiom colourless translucency to complete opacity. At 

 about 2 and 1 pound no colour can be detected. 



From these experiments I deduce the follov/ing conclu- 

 sions : 



(1.) Steam in its purely gaseous form is, as commonly sup- 

 posed, colourless, at least at small thicknesses. 



(2.) The orange red colour of steam by transmitted light 

 appears to be due to a particular stage of the condensing pro- 

 cess. Before condensation, steam is colourless and transpa- 

 rent ; it is next transparent and smoke-coloured; finally it 



