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



(3.) The state of tension of the steam seems only to affect the phenomenon 

 in so far as it renders the critical colorific stage of condensation more or less 

 completely oljservable. 



(4.) The absorptive action of steam on the spectrum is not exerted in the 

 same way as that of other gaseous coloured bodies, such as nitric acid gas and 

 iodine vapour. It cuts off", howevei-, totally the same part of the spectrum as 

 nitrous acid does. Its phenomena perhaps have a greater analogy to those of 

 opalescence than any other. 



The effect of mere change of mechanical structure in altering the optical 

 properties of bodies, is a phenomenon likely to give important information, both 

 as to the constitution of matter, and the constitution of light ; and the present 

 observation may perhaps be one day received as a contribution towards a mecha- 

 aical theory of vapour, including that most singular stage which intervenes be- 

 tween the gaseous and completely liquid form, and which is probably connected 

 with the mechanical suspension of clouds. It is at all events very important to 

 know that a portion of watery vapour confined in a close vessel, and subjected to 

 change of temperature alone, without chemical change, is capable of undergoing 

 the alterations of colour and transparency which have been adverted to. The 

 singular fact noticed by Sir D. Brewster in the case of nitrous acid gas, whose 

 colour deepens to an intense orange-red by the simple application of heat, seems 

 to be a fact of the same kind. 



I cannot doubt that the colour of watery vapour under certain conditions, is 

 the principal or only cause of the red colour observed in clouds. The very fact 

 that that colour only appears in the presence of clouds, is a sufficient refutation 

 of the only explanation of the phenomena of sunset and sumise having the least 

 plausibility, given by optical wi-iters. If the red light of the horizontal sky were 

 simply complementary to the blue of a pure atmosphere, the sun ought to set red 

 in the cleai'est weather, and then most of all. But experience shews that a lurid 

 sunrise or sunset is always accompanied by clouds, and in a great majority of 

 cases, when the changing state of previously transparent and colom-less vapoiu: 

 may be infen-ed from the succeeding rain. In like manner terrestrial Mghts seen 

 at a distance grow red and dim, when the atmosphere is filled with vapom- soon 

 to be precipitated. Analogy applied to the preceding observations would cer- 

 tainly conduct to a solution of such appearances ; for I have remarked that the 

 existence of vapour of high tension is by no means essential to the production of 

 colour, though of course a proportionably greater thickness of the medium must 

 be employed to produce a similar effect when the elasticity is small. 



Glasgow, 29rt December 18.38. 



