212 



NA TURE 



[December 28, 1893 



raised into position. The pipette with the crook is then taken 

 out of the fluid, and inverted. The crook is laid aside, and the 

 outside of the pipette cleaned with blotting-paper, by which all 

 adhering foreign grains are removed. The pipette now con- 

 tains a pure gathering of the mineral required, and it only 

 remains to discharge its contents, and this is of course accom- 

 plished by pushing down the piston. 



Minerals may thus be removed from every zone of a diffusion 

 ..jlumn ; and all the species which enter into the composition of 

 a reck, except of course the very heaviest, may be separately 

 obtained, with their specific gravity determined as an incident 

 of the process. 



In this absurdly simple fashion "getting it out" ceases to be 

 a penance, and becomes a pleasure. W. J. Sollas. 



THE CLOUDY CONDENSATION OF STEAMS 



'T^HE 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 car- 

 bonic 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 pro- 

 portion, 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 propor- 

 tion of water becomes saturated is called the " dew point." 



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. Vou 

 will observe that although heavily laden with water vapour the 

 air is perfectly transparent. If, now, we turn a tap, and so con- 

 nect the globe with an 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, which fills the globe and is perfectly visible as a cloud or 

 mist. In a few minutes ihe 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. 



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 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 compeasated 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. 



Then comes the question, why should the burnt air from the 

 flame so greatly increase the number of the condensed drops? 

 An answer, though perhaps not quite a complete one, is fur- 

 nished by some remarkable experiments made by M. Coulier, a 

 French pro fessor, nearly twenty years ago. He believed his ex- 

 periments showed that water vapour would not condense at all, 

 even at temperatures far bilow the dew point, unless there 

 were present in the air a number of material particles to serve 

 as nuclei around which the condensation would take place. All 

 air, he says, contains dust ; and anything that increases the 

 number of dust particles in the air increases 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, 



1 Extracted from a lecture on " Fogs, Clouds, and Lightning," delivered 

 at the Royal Institution on May 5. 



NO. 



I 26 I, VOL. 49] 



ard 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 fil- 

 tered through tightly packed cotton woo), is found to be per- 

 fectly 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. 



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 aclive 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 a 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. 



Every one has noticed how dense and dark a thundercloud is. 

 It shuts out daylight almost as if it were a solid substance, and 

 the glimmer that penetrates it is often imbued with a lurid or 

 copper-coloured tint. 



I had always found it rather difficult to believe that these 

 peculiarities were due simply to the unusual extent and thick- 

 ness of the clonds, as is commonly supposed to be the case, and 

 it occurred to me about three years ago, that perhaps some clue 

 to the explanation might be aff'orded by the electrification of a 

 jet of steam. On making the experiment I found that the 

 density and opacity of the jet were greatly increased when an 

 electrical discharge was directed upon it, while its shadow, if 

 cast upon a white screen by a sufficiently strong light, was of a 

 decidedly reddish-brown tint. 



As a possible explanation of the effect I suggested that there 

 might occur some action among the little particles of water of a 

 similar nature to that observed by Lord Rayleigh in his experi- 

 ments upon water jets. A jet of water two or three feet long is 

 made to issue in a nearly vertical direction from a small nozzle. 

 At a certain distance above the nozzle the continuous stream is 

 found to break up into separate drops, which collide with one 

 another, and again rebounding, become scattered over a con- 

 siderable space. But when the jet is exposed to the influence of 

 an electrified substance, such as a rubbed stick of sealing-wax, 

 the drops no longer rebound after collision, but coalesce, and the 

 entire stream of water, both ascending and descending, becomes 

 nearly continuous. 



There is one other point to which I wish to direct your par- 

 ticular attention. If the sealing-wax, or better, the knob of a 

 charged Leyden jar, is held very close to the jet, so that the 

 electrical influence is stronger, the separate drops do not coalesce 

 as before, but become scattered even more widely than when no 

 electrical influence was operating. They become similarly 

 electrified and, in accordance with the well-known law, repel 

 one another. 



We will now remove the water jet, and in its place put a little 

 apparatus for producing a jet of steam. It consists of a half- 

 pint tin bottle, through the cork of which passes a glass tube 

 terminating in a nozzle. When the water in the bottle is made 

 to boil, a jet of steam issues from the nozzle, and if we observe 

 the shadow of the steam jet upon the screen we shall see that 

 it is of feeble intensity and of a neutral lint, unaccompanied by 

 any trace of decided colour. A bundle of needles connected 

 by a wire with the electrical machine is placed near the base of 

 the jet, and when the machine is worked electricity is discharged 

 into the steam. A very striking effect instantly follows. The 

 cloud of condensed steam is rendered dense and dark, its 

 shadow at the same time assuming the suggestive yellowish- 

 brown colour. 



I at first believed that we had here a repetition, upon a smaller 

 scale, of the phenomenon which occurs in the water jet. The 

 little particles of condensed water must frequently come into 

 collision with one another, and it seemed natural to suppose 

 that, like Lord Rayleigh's larger particles, they rebounded under 

 ordinary circumstances, and coalesced when under the influence 



