418 MR JOHN AITKEN ON THE 



reduced to about half an atmosphere, or less, by means of an air-pump. Communication 

 is then opened between it and the glass receivers, which at present we will suppose to be 

 full of pure air, or, at least, ordinary unpolluted air, and also that the insides of the 

 receivers are wet. When the communication is made between the vacuum receiver and 

 the glass receivers, condensation takes place as usual in the air in the flasks on what dust 

 particles may be present. The stopcocks between the glass receivers, and between them 

 and the metal vacuum receiver, are now closed, and the airs containing the nuclei we 

 wish to experiment on are allowed to rush in and fill different glass flasks. This is done 

 by connecting the other pipe of the glass receiver with a metal pipe which terminates in 

 an inverted funnel for collecting the products from the different flames we wish to test. 



Suppose we admit the products of combustion from a paraffin-lamp to one of the 

 receivers, while we fill up the other with the products from a gas-flame in which a little 

 sulphur is being burned. This being done, the stopcocks for admitting the different airs 

 are now closed, and the stopcocks on the pipe connecting the two receivers are opened. 

 The metal vacuum receiver having been pumped down, its stopcock is slowly opened and 

 the contents of the two glass receivers are slowly and simultaneously expanded. When 

 this is done, fogging takes place in both receivers, but the air containing the sulphur 

 products is much denser than the other. It might be thought that the less denseness of 

 the paraffin products was due to a want of nuclei. That this is not the case is now shown 

 by again filling this receiver with more air from the lamp, but this is not necessary, 

 as ordinary air will do quite well, there being plenty of nuclei to stand dilution. After 

 the receiver is filled, the stopcock by which the air entered is closed, and the stopcock 

 connecting it with the metal vacuum receiver is quickly turned full on, and the air very 

 rapidly expanded. When this is done the products from the lamp give a very dense 

 fogging, which, for a very short time, looks as dense as that given by the sulphur 

 products in the other receiver. But note the difference in the behaviour of the two fogs. 

 The lamp products will clear rapidly. In a few seconds a visible decrease will take place 

 in its density, and on examining the upper part of the receiver it will be seen that the 

 particles are falling and leaving a clear space at the top. A little later this fogless space 

 will be found to have increased in depth, and the fog low down will at the same time 

 have greatly decreased in density. In a short time the fog is all gone, having rained 

 itself to the bottom of the receiver. Not so the other fog : it still persists, and is 

 nearly as dense as it was at first, and will remain dense for an hour afterwards ; and not 

 only so, but even after that time it shows no signs of clearing at the top, the fog still 

 persisting all through the air in the receiver, where it may still be detected hours after- 

 wards. The fog in this receiver illustrates the characteristics of a town fog ; that in 

 the other the characteristics of a country fog. 



The above experiment requires to be interpreted with caution. All the effects are 

 not what they at first appear to be. It will, therefore, be as well that we point out the 

 different influences in action helping to produce the appearances observed. We said that 

 slow cooling and condensation give a thin form of clouding, and quick condensation a 



