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1. On the Colour of Steam under certain circumstances. By 

 Professor Forbes. 



The author accidentally remarked, that the colour of the Sun 

 seen through vapour issuing from the safety-valve of a locomotive 

 engine is deep red, exactly similar to that which a column of smoke 

 or a smoked glass gives to it. 



He next noticed that this colorific character of steam extended 

 but a short way beyond the orifice, and that it gradually became 

 more opake, and perfectly white like noon-day clouds, both for 

 transmitted and reflected light. At moderate thicknesses, in this 

 state, its opacity is complete. 



These observations were fully confirmed by direct experiments 

 on high-pressure steam, made at Glasgow in December last. At 

 the moment of issuing from the steam-cock, it is perfectly trans- 

 parent and colourless ; at some distance from the orifice it becomes 

 transparent and orange-red ; but, still farther off, it is white, and 

 merely translucent. Tliese properties were traced in steam from 

 a pressure above that of the atmosphere of 53 lb., down to an ex- 

 cess of oidy three or four ; and as in all cases the redness of the 

 transmitted light was more or less distinctly seen, (and an excess 

 of 10 or 15 lb. does as well as any higher pi-essure), it was con- 

 cluded that the effect of partial condensation in producing the phe- 

 nomenon, would be rendered visible in great thicknesses of vapour 

 of the lowest tension. 



The great analogy of the colour of steam to that which the 

 clouds assume at sunset, or distant lights in certain conditions of 

 the atmosphere, lead the autlior to suggest this singular property 

 of condensing vapour as the probable cause of tliose phenomena, of 

 which no satisfactory explanation could be given whilst this fact 

 remained unknown. The prognostics of weather derived from the 

 colours of the sky also receive elucidation from the fact. 



Judging from the similarity of colour of steam and that of nitrous 

 acid gas, and the remarkable power of absorbing certain definite 

 rays of the spectrum discovered in that gas by Sir David Brewster, 

 the author thf>ught it probable that similar lines might be discover- 

 ed in the spectrum formed by light transmitted through steam, 

 and that these might be found to coincide with ihe atmospheric 

 lines of the spectrum noticed by the same philosopher. The ex- 

 periment was made with great care, but the expected result has 



