Steam under certain circumstances. 123 



cilities, Mr. Edington of the Phoenix iron-works at Glasgow 

 most kindly put at my disposition an excellent high-pressure 

 boiler, and further afforded me every facility for prosecuting 

 my experiments on the optical properties of steam. I first ex- 

 amined the simple pheenomena 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 boi'e of ^ inch. 

 When the safety valve (which acted with great promptness) 

 was loaded with 50 pounds 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 the height of a few inches above the cock, 

 and I'apidly 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 point therefore I resolved to transmit the light and to 

 analyse by a prism. A theodolite and good prism in front of 

 the telescope were placed at the distance of about 25 feet from 

 the boiler ; beyond the steam-cock a lantern with a lens for pa- 

 rallel rays was adjusted, and between the steam-cock and the 

 prism a slit of variable width. The 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 ad- 

 justment of the apparatus, and also for the purpose of con- 

 trast, I had provided a bottle, about 5 inches in diameter, full 

 of remarkably dense nitrous acid gas, which Mr. Kemp was 

 so good as to prepare for me. When this was placed where 

 the steam was to issue, the appearance of the nitrous acid spec- 

 trum was magnificently displayed. I then I'emoved the bottle 

 and opened the steam-cock gradually (the pressure on the 

 safety-valve being 55 pounds 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 

 no lines were visible in the remaining part. When the cock was 

 fully opened the spectrum exhibited a singular appearance ; 

 the bright red was the only part which seemed natural. The 

 extreme red was slightly 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 disagreeable hue, which I de- 

 scribed in a memorandum at the time as " dingy, alterna- 

 ting between yellow and purple, with shades of green ; 

 when the steam had its highest pressure there was a deci- 

 dedly purple tinge." The appearance to the naked eye 

 of the slit was now identically the colour of the nitrous acid 

 * The experiments were performed at night. 



