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



from the phenomena of absorption of light by gaseous bodies, and especiaUy the 

 singular action of nitrous acid gas in dividing the spectrum into a vast num])er of 

 liands, discovered by Sir D. Brewster, I thought it by no means improbable that 

 steam, acting in a similar manner, might exercise its specific action upon the 

 prismatic colours at many points. Should this conjecture be confirmed, I also 

 foresaw an application to the phenomena of the atmosphere and the production 

 of the atmospheric lines of the solar spectrum, also remarked by Sir D. Brewster. 

 After various ineffectual attempts to obtain the requisite facilities, Mr Eding- 

 ton, of the Phoenix Iron Works at Glasgow, most kindW ]>ut at my disposition an 

 excellent high pressure boiler, and farther afforded me every facility for prosecu- 

 ting my experiments on the optical properties of steam. I first examined the 

 simple phenomena of colour as seen by the naked eye. A lantern * was held 

 behind a jet of steam, issuing from a stopcock in the top of the boiler, having a 

 bore of ^ inch. When the safety-valve (which acted with great promptness) was 

 loaded with 50 lb. on the inch, the steam issued nearly invisible, and, at the small 

 thickness of the jet in that part, perfectly colourless. As the light was raised, 

 the orange colour appeared at a height of a few inches above the cock, and rapid- 

 ly deepened up to a height of about 20 inches ; after which it appeared that the 

 rapid condensation of the steam only rendered it more opake, without deepening 

 its hue. At that height, therefore, I resolved to transmit the light, and to ana- 

 lyze it by a prism. A theodolite, and good prism in front of the telescope, were 

 placed at a distance of about 25 feet in front of the boiler. Beyond the steam- 

 cock a lantern, with a lens refracting parallel rays, was adjusted, and between the 

 steam-cock and the prism a slit of variable width. Tlie light, reaching the prism 

 through the slit, must first pass through the column of steam at a height of about 

 20 inches from the orifice. To test the adjustment of the apparatus, and also 

 for the purpose of contrast, I had provided a bottle, about five inches diameter, 

 full of remarkably dense nitrous acid gas, which Mr Kemp was so good as to pre- 

 pare. When this was placed where the steam was to issue, the appearance of the 

 nitrous acid spectrum was magnificently displayed. I then removed the bottle, 

 and opened the steam-cock gradually (the pressure on the safety-valve being 

 55 lb. above the atmosphere, or the tension of the steam 4f atmospheres.) The 

 violet end of the spectrum was almost instantly absorbed, then the whole blue, 

 and part of the green, just as in the nitrous acid spectrum, but 7io lines mere 

 visible in the 7^emaining part. When the cock was fully opened, the spectrum ex- 

 hibited a singular appearance. The bright red was the only part which seemed 

 natural. The extreme red was sliglitly invaded by the opacity of the steam. 

 Most of the orange, the yellow, and as much of the green as was not absorbed, 

 had a dirty and disagreeable hue, which I described in a memorandum at the time 



* The experiments were performed at night. 



