VAPOUR, OPALESCENT. This appears to be the most con venient appellation for what has sometimes been termed red or orange steam. It is a condition of condensed and condensing aqueous vapour which was first distinctly recognised, and its optical properties inveis tigated by Dr. James 1). Forbes, F.R.S., when Professor of Natural Philosophy in the University of Edinburgh. Its effects in nature have been observed from time immemorial, though ascribed to other causes; and there can be no doubt that it would long ago have been recognised and described by those practically conversant with steam and steam-engines, had not its effect on luminous bodies been confounded with that of smoke, from which it Is in fact undistinguishable by the eye. In the year 1838, Professor Forbes, standing near a locomotive engine which was discharging a large quantity of high-pressure steam by its safety-valve, chanced to look at the sun through the ascending column of vapour, and was struck by 'geeing it of a very deep orange colour, exactly similar to dense smoke, or to the colour imparted to the sun when viewed through a common smoked glass. The !ionic he found might be observed during the ordinary progress of the engine in the steam thrown into the chimney, but the presence of smoke itself rendered the experiment less satisfactory. He afterwards observed that while for some feet or yards from the safety-valve at which the steam blows, its colour for transmitted light is the deep orange red, at a greater distance, the steam being more fully condensed, the effect entirely awes. Even at moderate thicknesses the steam cloud is absolutely opaque 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 Indeed, according to Professor Forbes, have a perfect of structure. When the steam is in this state no r is perceptible in purling from the thickness cones peeling to translucency to that which is absolutely opaque.
Protestor Forbes proceeded to investigate this novel subject by means of a high-pressure steam-boiler, and a theodolite with a good prism placed in front of the telescope, and from the experiments he made deduced the following conclusions :—" 1. Steam In its purely gaseous form is, as commonly supposed, colourless, at least in small tiuckneeses. 2. The orange-red colour of steam by transmitted light appears to be due to a particular stage of the condensing process. Before condensation steam is colourless and transparent ; it is next transparent and smoke coloured ; finally it becomes colourless at small thicknesses, and absolutely opaque at greater. 3. The state of tension of the steam teems only to affect the phenomena so far as it renders the critical colorific stage of condensation more or less completely observable. 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 nitrous acid gas, and iodine vapour. It cuts off, however, 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." l'hiL Mag.; series 2, vol. xiv., p. I21-126; the paper having been read before the Royal Society of Edinburgh, on January 21, 1839.
The combination of a variety of other facts with those thus made known by Professor Forbes, relating to the nature and properties of condensing steam with respect to light, have subsequently led other men of science to unite in the conclusion, that the structure of orange steam is in reality that of opalescent bodies, with the phenomena of which, as we have seen, he recognised the analogy of those presented by it. Innumerable globules of water are formed throughout the still gaseous vapour, by the joint action of which on light the colour of the aggregate mass is produced. It is on this account that its absorptive action on the spectrum is not exerted in the same way as nitrous acid gas and iodine vapour, which themselves posaess true colour. The other results obtained by Professor Forbes also harmonise with this conclusion.
But though we are thus obliged to relinquish the idea that aqueous vapour in any state is itself truly coloured, the observation of the existence of this particular condition of a condensing volume of steam is of great value ; especially in its application to the phenomena of nature. Professor Forbes at once inferred from his investigation of it, that the condition of watery vapour he had observed " is the principal or only cause of the red colour observed in clouds ; " and ho is entitled to the credit of being, in fact, the discoverer of the true cause of the colours of dawn and sunset. In a subsequent elaborate communication On the Colours of the Atmosphere,' ho investigated the history of science on this subject, refuting the fallacious inferences which had prevailed, and applied to it his own observations. "Soon after the maximum temperature of the day, and before sunset," ho remarks, " the surface of the ground, and likewise the strata at different heights in the atmosphere, begin to lose heat by radiation ; this is the cause of the deposition of dew, and consequently in severe weather we have vast tracts of air containing moisture in that critical state which precedes condensation,"—in other words, in the red opalescent condition, by which the rays of the setting sun are coloured accordingly. For the details of this subject we must refer to Professor Forbes's paper in tho Trans. of tho Royal Society of Edinburgh,' voL xiv. But we cannot wholly agree with him as to the colours of the morning sky, the phenomena of which appear to us to be the same with those of the evening, but in the reverse order. Mr. Luke Howard had long before observed the connection of the presence in the sky of a stratum of vapour having the peculiar red colour, with the coming or actual formation of dew, and had given to it the name of the dew-band, of which many observations will be found in his Climate of London.'