371 



XVIII. — On the Colour of Steam under certain circumstances. By James D. 

 Forbes, Esq., F.R.SS.L. &f Ed., Professor of Natural Philosophy in the Univer- 

 sity of Edinburgh. 



Read 21st January 1839. 



In the end of May, or beginning of June last, I happened to stand near a loco- 

 motive engine on the Greenwich Railway, which was discharging a vast quantity 

 of high-pressure steam by its safety-valve. I chanced to look at the sun through 

 the ascending column of vapom*, and was struck by seeing it of a very deep 

 orange-red colour, exactly similar to that of dense smoke, or the colour imparted 

 to the sun when viewed through a common smoked glass. 



I did not pay much attention to the fact at the moment, nor attempt to vary 

 the experiment ; but, reflecting on it afterwards, it seemed to me not only as in 

 itself very singular, but as still more extraordinary, because I had never heard 

 of a property of steam which must have been witnessed by thousands of per- 

 sons. Some months after (in the end of October), being on the Newcastle and 

 Carlisle Railway, I resolved to verify the fact, which I had no difficulty in doing, 

 and I farther discovered a very important modification of it. For some feet or 

 yards from the safety-valve at which the steam blows off", its colour for transmit- 

 ted light is the deep orange-red I have described.* At a greater distance, how- 

 ever, the steam being more fully condensed, the effect entirely ceases. Even at 

 moderate thicknesses, the steam-cloud is absolutely opake to the direct solar rays : 

 the shadow it throws being as black as that of a dense body, and when the thick- 

 ness is very small, it is translucent, but absolutely coloiu-less, just like thin clouds 

 passing over the sun, which have, indeed, a perfect analogy of structure. When 

 the steam is in this state, no indication of colour is perceptible in passing from the 

 thickness corresponding to translucency, to that which is absolutely opake. 



Having made these observations, which were aU that the circumstances enabled 

 me to accomplish, I was very anxious to verify them under steam of various 

 pressures, and to determine the following, amongst other points. (1.) Whether 

 steam, in its pm-ely gaseous form, is really, as commonly supposed, colourless ; 

 (2.) Whether the colour depends on a stage in the process of condensation, and 

 on that alone ; (3.) What effect the tension of the steam has upon the phenomena. 



But there was another inquuy which interested me much more than all these, 

 which was, to examine how the spectrum was affected by the absorbent action of 

 the steam, which appeared to leave the red and orange rays predominant. Judging 



* The same may be observed, during the ordinary progress of the engine, in the steam thrown into 

 the chimney, but the presence of smoke renders the experiment less satisfactory. 

 VOL. XTV. PART II. 3 B 



