May 26, 1892] 



NA TURE 



91 



prevent something which takes place in the jet under ordinary 

 conditions. The particles in a jet being in rapid movement, 

 there are frequent collisions, and consequent coalescence of the 

 drops, but when the particles are electrified they repel each 

 other, and coalescence is prevented. 



The jet on becoming dense emits a peculiar sound, which is 

 the same whatever be the cause of the increased density. But, 

 when electrified, along with this sound there is another, due to 

 the discharge of the electricity, which causes the electrified jet 

 to appear to make a louder noise. The jet, instead of changing 

 suddenly in appearance when electrified, may be made to change 

 very gradually, either by electrifying it by means of a very sharp 

 point, or by aiding the discharge by a flame. Under these con- 

 ditions, the jet emits only the sound produced when dense from 

 any of the other causes. 



2nd. A Large Number of Dust Particles in the Air. 

 Flame has not been found to have any influence on the steam 

 jet, but on bringing the products of combustion to the jet, it at 

 once becomes dense, and remains dense so long as the supply is 

 kept up, and the jet has exactly the same appearance as if 

 electrified. When in this condition electricity does noi increase 

 its density any further. The increased density is here due to 

 the large number of dust nuclei, causing a great increase in the 

 number of water drops, and these being very small, they will 

 have less independent movement, and therefore fewer collisions, 

 and the reduction in number from this cause will therefore be 

 very slight. 



3rd. Low Temperature of the Air. 



When a steam jet condenses in air at ordinary temperatures 

 it has but little density, but, if the open end of a metal tube 

 cooled to 45° be held near the origin of the jet, the condensa- 

 tion at once becomes dense, and neither electrification nor an 

 increased supply of nuclei makes it any denser. In a room at 

 a temperature of 46° the jet is always dense, and neither elec- 

 tricity nor the products of combustion have any effect on it, but 

 when the temperature rises to 47^ the jet begins to get a little 

 less dense, and electricity now increases its density slightly. At 

 50° the jet is much thinner, and both electricity and the pro- 

 ducts of combustion have a marked effect on it. The change 

 produced by the cold air cannot be entirely due to the lower 

 temperature causing more vapour to be condensed, as the fall of 

 temperature is slight, while the increase in density is great. 

 The increased density is shown to be due to a change which 

 takes place in the fifms of the small drops with the fall of 

 temperature. When the temperature is above a certain point, 

 the films have no repulsive action, and the drops coalesce on 

 collision ; whereas when cooled below a certain temperature 

 the well-known repulsion comes into play and prevents 

 coalescence. This was proved by repeating Lord Rayleigh's 

 experiments with water jets. When the temperature of the 

 water was over 160°, the drops had no tendency to scatter, and 

 the presence of an electrified body had no influence on the jet. 

 It was only when the temperature fell that the scattering began, 

 and the electrical disturbance produced coalescence. The effect 

 of the low temperature is the same as that of electrification : 

 both of them prevent the water drops coming into contact, one 

 by electrical repulsion, and the other by the repulsive action of 

 the water films, and the result is the same— namely, an increase, 

 or rather a prevention of the decrease, in the number of the 

 particles, and a consequent increase in the density of the 

 clouding. 



4th. High Pressure of the Steam. 



Below a temperature of 46"^ the jet is dense at all pressures, 

 and as the temperature rises the density decreases, but the 

 density may be made to return by increasing the pressure. The 

 increased density of the high-pressure steam jet is due to an 

 increase in the number of drops produced, (i) by the jet being 

 more cooled by the greater amount of air taken into it ; (2) 

 by a larger supply of dust nuclei ; and (3) owing to the rate 

 at which the condensation is made to take place, a larger 

 number of dust particles are forced to become centres of con- 

 densation. 



Sth. Rough Nozzles and Obstruction in Front of the Nozzle. 



Rough nozzles and obstruction in front of the nozzle are found 

 to act in the same way as increase of pressure : they aid pres- 

 sure in producing its effects with a less velocity of steam. They 

 act by producing eddies, which mix more air with the steam, so 



NO. I I 78, VOL. 46] 



lowering the temperature of the jet, increasing the number of 

 dust nuclei, and quickening the rate of condensation. 



The seat of sensitiveness to all these influences causing the 

 condensation in the jet to become dense is near the nozzle. Both 

 low temperature and obstructions have an effect only when they 

 act verj' close to the nozzle ; while electricity and increase in 

 number of nuclei have a slight, but rapidly diminishing, influ- 

 ence to a distance of 3 or 4 cm. from the nozzle. 



Part II. 

 Colour Phenomena connected with Cloudy Condensation. 

 The manner in which cloudy condensation changes after it is 

 formed is pointed out, and it is also shown that the number of 

 dust particles which become centres of condensation depends on 

 the rate at which the condensation is made to take place, slow 

 condensation producing few water particles and thin clouding, 

 while quick condensation produces a large number of water 

 particles and dense clouding. It is only when the dust particles 

 are few that all of them become active centres of condensation. 



Colour Phenomena in Steam Jets. 



Colour has been seen by Principal Forbes and others in the 

 steam escaping from engine boilers, but these colour phenomena 

 have as yet been but little studied. For observing the colour in 

 steam jets, the author has found it to be a great advantage to 

 inclose the jet in a tube, and examine the effect through some 

 length of condensing steam. Steam by itself has no colour in 

 moderate lengths, but when mixed with a certain amount of cold 

 air, and a certain quantity of dust, very beautiful colours are 

 produced. A jet of steam is allowed to blow into the open end 

 of a tube, and the amount of dusty air entering with it is 

 regulated to the necessary amount. When the jet is condens- 

 ing under ordinary conditions, the colour of the transmitted 

 light varies from greens to blues of various depths, according to 

 the conditions. The colour may be made very pale or extremely 

 deep by varying the conditions. If the condensation in the jet 

 be made to change and become dense by any of the influences 

 already mentioned, the colour changes, and generally becomes 

 of a yellowish-brown. 



This yellow colour, seen through steam when the jet is electri- 

 fied, has been previously observed. It was thought that the 

 colour was due to the electricity, and that the experiment ex- 

 plained the lurid colour of thunder-clouds. There does not, 

 however, seem to be any connection between the electrification 

 and the colour, as the transmitted light becomes of the same 

 Ivnid hue when the jet is made dense by any of the other in- 

 fluences. 



The yellow colours seen through steam are not generally so 

 fine as the greens and blues, but when the density is due to high- 

 pressure steam the yellow is very fine. 



Colours in Cloudy Condensation produced by Expansion. 



No colours had been previously seen in the light transmitted 

 directly through the clouded air produced by expanding saturated 

 air in a receiver. It was thought this was due to the slownes^s 

 with which this process is generally made to take place in the 

 expansion experiments. On arranging an experiment to make 

 the rate of condensation quick, beautiful colours were seen on 

 looking through the clouded air. An air pump was connected 

 with a metal tube provided with glass ends. The capacity of 

 the tube was small compared with the capacity of the receivers 

 usually used in these experiments. When the air in the tube 

 was suddenly expanded, the light passing through it became 

 beautifully coloured, and the colour, and the depth of the 

 colour, varied with the conditions. With few dust particles in 

 the air, a slight expansion made the transmitted light blue ; a 

 greater expansion changed it to green, and then to yellow ; and 

 when the expansion was still further increased the colour 

 changed, and a second blue made its appearance, followed by a 

 second green and yellow. But, if very many particles were pre- 

 sent, the same amount of expansion which produced the second 

 yellow only gave a very deep blue. When it is desired to 

 produce these colour phenomena on a large scale a vacuum 

 receiver is used. This receiver is connected with the experi- 

 mental tube or flask by means of a pipe fitted with a stop-cock. 

 After a partial vacuum has been made in the receiver, the con- 

 nection between it and the flask or tube in which the colours 

 are to be shown is suddenly opened, when the colour-produc- 

 ing condensation is produced. These colour phenomena fade 



