PROFESSOR FORBES ON THE COLOUR OF STEAM UNDER CERTAIN CIRCUMSTANCES. 373 



as " dingy, alternating between yellow and purple, with shades of green. When 

 the steam had its highest pressure, there was a decidedly purple tinge." The 

 appearance to the naked eye of the slit was now identically the colour of the ni- 

 trous acid gas, through which I from time to time viewed a distant gas flame, 

 and compared it with the colour of the slit. The experiment was performed 

 under 50 and 55 lb. many times over. The light examined was then caused to 

 pass through the steam only 10 inches above the orifice of the stop-cock, under 

 the idea that though the colour there was fainter, possibly there might be a ten- 

 dency to develope lines in the spectrum. 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 shewed the violet 

 alone absorbed. * 



Steam blowing off at 25 lb. The lantern and slit 20 inches above orifice, as 

 at first. To the eye the light appears as red as under 551b. Mr Edington ob- 

 served, that the colour is deeper than that of the nitrous acid gas bottle. Neither 

 he nor his assistants ever observed the colour of steam before. Prismatic pheno- 

 mena as before, only the obscuration not quite so great. 



Steam blowing off at 15 lb. " Evidently redder than the gas bottle. Same 

 phenomena of spectrum, but green remains pure throughout, and verges on 

 (bounds immediately with) orange. During the absorption of violet before va- 

 nishing (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 

 five or six feet, and wherever it was not entirely obscured, it appeared of different 

 shades of smoke colour, up to an intense tawny orange. 



With 7 lb. on the inch, still visibly red to the eye : prismatic phenomena 

 similar, but slighter. 



With 4 lb., no longer visibly red to the eye, when arranged as above ; and 

 even 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 red- 

 ness close to the orifice, but every where above, the transition is from colourless 

 translucency to complete opacity. At about 2 and 1 lb., no colour can be detected. 



From these experiments I would deduce the following conclusions : 



(1.) Steam in its purely gaseous form, is, as commonly supposed, 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 process. In the incipient state of conden- 

 sation, steam is colourless and transparent ; it is next transparent and smoke- 

 coloured ; finally it becomes colourless at small thicknesses, absolutely opake at 

 greater. 



