122 Professor Forbes oji the Colour of 



terwards it seemed to me not only as in itself ver}' singular, 

 but as still more extraordinary that I should never have heard 

 of a property of steam which must have been witnessed by 

 thousands of persons. Some months after (in the end of Oc- 

 tober), being on the Newcastle and Carlisle railway, I resolved 

 to verify the fact, which I had no difficulty in doing, and 

 I further discovered a very important modification of it. For 

 some feet or yards from the safety valve at which the steam 

 blows, its colour for transmitted light is the deep orange red 

 I have described*. At a greater distance, however, the steam 

 being more full}' condensed, the eifect 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 thickness is very small 

 it is translucent, but absolutely colourless, 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 correspond- 

 ing to translucency to that which is absolutely opake. 



Having made these observations, which were all that the 

 circumstances enabled me to accomplish, 1 was very anxious to 

 verify them with steam of various pressures, and to determine 

 the following amongst other points: (1) whether steam in its 

 purely gaseous form is really, as commonly supposed, colour- 

 less; (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 phoenomena. 



But there was another inquiry 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 left 

 the red and orange rays predominant. Judging from the phie- 

 nomena of absorption of light by gaseous bodies, and especi- 

 ally the singular action of nitrous acid gas in dividing the spec- 

 trum into a vast number of bands, 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 coloui's at many points. Should this con- 

 jecture be confirmed, I also foresaw an application to the phae- 

 nomena of the atmosphere and the production of the atmo- 

 spheric lines of the solar spectrum also remarked by Sir D. 

 Brewster. 



After various ineffectual attempts to obtain the requisite fa- 



* The same may be observed daring the ordinary progress of the engine 

 in the steam thrown into the chimney, but the presence of smoke renders 

 the experiment less satisfactory. 



