WEEKLY EVENING MEETING, Friday, May 5, 1893. SIR FREDERICK BRAMWELL, Bart. D.C.L. LL.D. F.R.S. Honorary Secretary and Vice-President, in the Chair. SHELFORD BIDWELL, Esq. M.A. LL.B. F.R.S. M.R.I. Fogs, Clouds and Lightning. THE air, as every one knows, is composed almost entirely of the two gases, oxygen and nitrogen. It also contains small quantities of other substances, of which the chief are carbonic acid gas and water vapour, and it is the latter of these constituents, water vapour, or "steam" as it is sometimes called, that will principally concern us this evening. The quantity of invisible water vapour which the air can at any time take up depends upon the temperature; the higher the temperature of the air the more water it can contain. The proportion, however, never exceeds a few grains' weight of water to a cubic foot of air. Air at any temperature, containing as much water as it can possibly hold, is said to be "saturated," while the temperature at which air containing a certain proportion of water becomes saturated is called the "dew point." The water vapour contained in the atmosphere plays a very important part in many natural phenomena. Among other things, it is the origin of clouds and of fogs. If a body of air containing water in the form of invisible vapour is quickly cooled to a temperature below its dew point, a portion of the vapour becomes condensed into a number of minute liquid particles of water, forming a visible mist, which, when it is suspended in the upper regions of the air, is called a cloud, and when it rests upon the surface of the earth is only too familiarly known as a fog. The cooling of water-laden air may be brought about in various ways, resulting in the formation of clouds of several distinct characters. [Photographic examples of cumulus, stratus and cirrus clouds were exhibited upon the screen.] For experimental purposes a small body of air may be most conveniently cooled by allowing it to expand. I have here a flask of air which can be connected with the partially exhausted receiver of an air-pump. Inside the flask is an electrical thermometer or thermo-junction, the indications of which can be rendered evident to all present by the movement of a spot of light upon a scale attached to the wall. A deflection of the VOL. XIV. (No. 87.) H Royal Institution of Great Bruin WEEKLY EVENING MEETING, FREDERICK BRAYWELL, Bart D.CL. ILD. FRS Hy PROFESSOR DEWAR, MA LLD. FRS. KRI Liquid Atmospheric Air. prosecution of research at temperatures approaching the am of lute temperature is attended with difficulties and dangers of mary kind. Having no recorded experience to ge neting such investigations, the best instruments and methods of ing have to be discovered. The necessity of dengue kind of vessel for staring and manipulating exceedingly made like liquid oxygen and liquid air, because apprehe cal properties of the bodies came under eve Fils, being in active ebullition, were in a condition which madened cal measurements impossible. All attempte principle of using a succession of surmonding ples wek ular space between such vesels having the cool of the our coming from the boiling liquid led through them pred lure. Apart altogether from the rapit ebulitin meng th experimental work, the fact that it took place valved litional cost in the conduct of experiments on the properties f tter under such exceptional conditions of tape While suffering great anxiety on the question of expenditone, the smiths Company came forward with the hundem ti 1001 to continue the work with improved apparates Personally, sire to express my grateful thanks to the Golden endering such encouragement and suppert In careful consideration it became apparent that f theking the problem was to conduct is of the relative amounts of heat conveyed to boiling ly, by means of the convective trade particles, and, secondly, by radiation from vanning early experiments of Duling and Pear def proved the very important part played by the pap ding a body in dissipating beat he year 1873 I need a highly-t "On the Physical Cons of Bring ut XIV. (Ne 87) spot of light to the left indicates cold, to the right, heat. When the stop-cock is opened so that a portion of the air escapes from the flask into the air-pump receiver, you see at once a violent movement of the spot of light to the left, showing that the expansion of the air is accompanied by a fall of temperature. If more air from the room is allowed to enter the flask, the spot moves in the opposite direction. The large glass globe, upon which the beam from the electric lantern is now directed, contains ordinary air, kept in a state of saturation, or nearly so, by the presence of a little water. You will observe that although heavily laden with water vapour the air is perfectly transparent. If, now, we turn tap and so connect the globe with the exhausted receiver, the air expands and becomes colder; the space inside the globe is no longer able to hold the same quantity of water as before in the form of vapour, and the excess is precipitated as very finely divided liquid water-water dust it may be called-which fills the globe and is perfectly visible as a cloud or mist. In a few minutes the cloud disappears, partly, no doubt, because some of the particles of water have fallen to the bottom of the vessel, but chiefly because the air becomes in time warmed up to its original temperature (that of the room), and the suspended water is converted back again into invisible vapour. Now let us repeat the experiment, and before the cloud has time to disperse let us admit some fresh air from outside; the cloud, as you see, vanishes in an instant. The compression of the air raises the temperature above the dew point, and the small floating particles of water are transformed into invisible vapour. I once more rarefy the air, and admit a fresh supply while holding the flame of a spirit-lamp near the orifice of the inlet pipe, so that some of the burnt air is carried into the interior of the globe. When the air is again expanded a cloud is formed which is, as you observe, far more dense than the others were. It appears on examination that the increased density of this cloud is not due to the condensation of a greater quantity of water. Little, if any, more water is precipitated than before. But the water particles are now much more numerous, their increased number being compensated for by diminished size. Within certain limits, the greater the number of particles into which a given quantity of water is condensed, the greater will be the apparent thickness of the mist produced. A few large drops will not impede and scatter light to the same extent as a great number of small ones, though the actual quantity of condensed water may be the same in each case. An Then comes the question, why should the burnt air from the flame so greatly increase the number of the condensed drops ? answer, though perhaps not quite a complete one, is furnished by some remarkable experiments made by M. Coulier, a French professor, nearly twenty years ago. He believed his experiments pointed to the conclusion that water vapour would not condense at all, even at temperatures far below the dew point, unless there were 66 present in the air a number of material particles to serve as nuclei around which the condensation could take place. All air, he says, contains dust; by which term he does not mean such dust as is rendered evident in this room by the light scattered along the track of the beam issuing from the electric lantern, which consists of comparatively gross lumps of matter, but particles of ultra-microscopical dimensions, more tenuous than the motes seen in a sunbeam." It is upon such minute specks of matter that water vapour is condensed. Anything that increased the number of dust particles in the air increased the density of the condensation by affording a greater number of nuclei. Air in which a flame had been burnt he supposed to be very highly charged with finely-divided matter, the products of combustion, and thus rendered extraordinarily "active" in bringing about condensation. And that, according to Coulier's view, is the reason why such a dense fog was formed when air which had been contaminated by the spirit flame was admitted to our globe. On the other hand, air, even burnt air, which has been filtered through tightly packed cotton wool, is found to be perfectly inactive. No cloud or mist will form in it, however highly it may be supersaturated. Coulier explained this fact by supposing that the process of filtration completely removed all dust particles from the air. On the table before you is a globe containing air which has been thus treated, and which is kept saturated by a little water. When this globe is connected with the exhausted receiver, no trace of any mist is produced: the air remains perfectly clear. We will now admit a little of the ordinary air from outside, and again cool it by expansion. Quite a respectable cloud is thereupon formed in the globe. The experiments of Coulier were repeated and confirmed by Mascart. The latter also made one additional observation which may very probably turn out to be of great importance. He found that ozone, or rather, strongly ozonised air, was a very active mist producer, and that unlike ordinary air, it was not deprived of its activity by filtration. Four or five years later, all the facts which had been noticed by Coulier, and others of an allied nature, were independently discovered by Mr. Aitken, who has devoted much time and study to them and made them the foundation of an entirely new branch of meteorology. Later, perhaps, we may see reason to doubt whether all the conclusions of Coulier and Aitken are quite accurate, especially as regards the action of so-called products of combustion. What has been said so far applies equally to the generation of clouds and of country fog, for a pure unadulterated fog, such as occurs in rural districts, consists simply of a cloud resting upon the surface of the earth. The fogs, however, which afflict many large towns, and London in a marked degree, appear to possess a character peculiar to themselves. They are distinguished by a well-known |