FOGS. 213 



saturated, while in ordinary air dust-particles form the nuclei on 

 which the vapour condenses. Since every cloud-particle is repre- 

 sented by a dust-particle, the more abundant the dust the more 

 dense the cloud. 



To return to the experiment. When steam was passed into 

 the receiver containing filtered air, no vapour condensed in its 

 visible form until circulation brought it into contact with the free 

 surfaces of the sides of the receiver, when it gradually condensed 

 on those free surfaces. The density of the fog in the receiver 

 containing ordinary air indicates what a large amount of dust is 

 present every day in the air around us. But the particles of fog 

 do not represent all the dust-particles in the air experimented 

 upon. They do not tell the whole truth. That the fog-particles 

 first produced on admission of the steam only represent a small 

 part of the dust-particles present may be demonstrated by the 

 following experiment : — 



Let as much steam be blown into a receiver containing ordi- 

 nary air as will produce a dense fog. Allow the fog to settle, but 

 do not allow any dusty air to enter. After the fog has settled, 

 blow in more steam. Again you will find a dense fog condensed 

 on the dust that escaped the first condensation. Allow this to 

 settle, and repeat the process a number of times, and you will find 

 that there is still fog forming. But after each condensation the 

 fog becomes less and less dense, till at last it ceases to be seen, 

 although on closely looking into the receiver the condensed 

 vapour will be seen falling as rain. 



When the steam was blown in the first time, the fog was very 

 fine-textured. Each particle was so small that it floated in the 

 air. After each condensation the fog became less and less dense. 

 It at the same time became more coarse-grained and heavier, and 

 was seen faUing slowly. Near the end no fog was visible ; 

 nothing but fine rain appeared to be falling. If the air was still 

 further purified, even the rain ceased. In fact, the conditions 

 that obtained in the receiver containing filtered air have been 

 brought about, — the air has become completely freed of dust. 



At this point it will be interesting to consider the effect of 



absence of dust. Every object on the face of the earth would, 



like the sides of the receiver that contained only filtered air, act 



New Series, Vol, I. 



1888, Q 



