416 



Mr. J. Aitken. On some Phenomena 



[Apr. 28, 



show signs of clearing. The clearing did not, however, take place 

 regularly ; one moment the jet was dense and the next it was ordi- 

 nary. These fluctuations would be due to the unequal temperature 

 of the air coming to the jet. At one moment the air would be the 

 air of the temperature of the room : the next it would be this air 

 slightly heated by the metal pipe and nozzle. So that when the jet 

 drew its supply of air horizontally its condensation was ordinary, and 

 when the air currents in the room prevented this heated air from 

 coming to the jet its condensation vras dense. 



A slight alteration was then made in the arrangement ; the jet 

 was now directed downward at the end of the horizontal pipe. By 

 this means the air heated on the pipe and nozzle was prevented from 

 mixing with the jet. The jet was directed at a small angle from the 

 vertical to prevent the hot air and vapour of the jet rising to the 

 nozzle. With this arrangement the following vras the result : up to 

 a temperature of 46° the condensation was dense, and neither elec- 

 tricity nor the products of combustion had any effect on the density ; 

 but when the temperature rose to about 47° electrification began to 

 have just a perceptible effect in increasing the density. At about 48° 

 the electricity had an easily observed effect, and the products of com- 

 bustion also had a slight effect. At a temperature of 50 = the jet 

 had become decidedly thinner, and both electricity and the products 

 of combustion had a decided effect in increasing its density. When 

 the temperature rose to 55° the jet lost its dense appearance, and both 

 electricity and the products of combustion had a very marked effect. 



It might be thought that by observing a steam jet in the open air 

 we could tell if the temperature of the air was above or below a 

 certain point. This, however, can only be done in a very rough way, 

 as the conditions are variable and not within our knowledge. We 

 would require to know the pressure of the steam, and the degree to 

 which the air was heated by the pipe. In a general way it may be 

 stated that in the open air a steam jet looks dense if the temperature 

 be below 50°, and ordinary if above 55°. But it is often difficult to say 

 what is ordinary and what is dense condensation, unless the observa- 

 tions are made carefully and by examining how close to the nozzle 

 the particles are visible. Of course if we could electrify the jet, 

 or supply it with the products of combustion, we could tell when- 

 ever the temperature was over or under 47°. 



The sudden alteration in the appearance of the jet when supplied 

 with air at a temperature of 46 3 points to some change in the in- 

 fluences in action in the condensing jet. The great increase in density 

 cannot be due to an increase in the amount of vapour condensed, as the 

 fall in the temperature is slight. Further, it will be observed that 

 the jet has ceased to be influenced by electricity, and by the products 

 of combustion. The only explanation I could think of was, that at 



