FORMATION OF SMALL CLEAR SPACTS IN DUSTY AIR. 269 



ridge, or up the steep sides. Now, suppose a stone to be rolled up the 

 ridge of this mountain, by a force acting in a direction along the ridge, 

 it is evident that if the stone gets off the ridge, that it will fall down the 

 quicker slope towards the side ; and if the stone keeps the ridge, and we 

 succeed in rolling it nearly to the summit, but there meet a slope too steep 

 for us to push the stone up, then the stone will obviously be in a position of 

 unstable equilibrium, and the slightest fall will cause it to leave the easy slope 

 of the ridge, and, once started on the quick descent of the sides, its motion will 

 be rapidly accelerated in a direction at right angles to that in which we are 

 pushing ; thus the stone will descend the quick slope of the side with great 

 velocity, even while the force which pushed it up the ridge is still acting on it. 

 The direction in which the force now acts on the stone is such that it no longer 

 tends to prevent it falling ; and further, supposing it was directly opposed to 

 its motion, it would have but little effect against the steep slope of the 

 side. Now draw the contour lines of this mountain. It will be found that 

 they exactly correspond to the isothermal lines round the hot wire placed 

 between the cold plates. The dust and the stone each fall towards the side, 

 because that is the direction of steepest slope. 



General Remarks. 



This tendency which the dust in our atmosphere has to move away from 

 hot bodies, and attach itself to cold ones, will, I have no doubt, help to explain 

 many phenomena which are not at present well understood. No doubt, many 

 things will suggest themselves to different minds as receiving their explanation 

 in this somewhat curious liking of dust for lodging in cold places. Among 

 other things, it explains the reason why stove and hot-air heated rooms are 

 always so much dirtier than those warmed by open fires. In a stove-heated room 

 the air is warmer than the walls and than the objects in the room, the dust there- 

 fore tends to leave the air, and to deposit itself on every object colder than 

 itself in the room ; whereas, in a room warmed with an open fire, the heating 

 being principally done by radiation, the walls and furniture are hotter than the 

 air, they therefore tend to throw off the dust, and eveu when it does fall on them, 

 it does not adhere with that firmness with which it does to a cold surface, and 

 any breath of air easily removes it. 



Diffusion also, no doubt, plays some part in determining whether dust 

 shall or shall not adhere to the walls and ceilings of rooms. 



Again, a knowledge of this tendency of dust to settle on cold surfaces is 

 necessary to enable us fully to explain why so much soot adheres to the inside 

 surfaces of chimneys. If the smoke were cold, so much soot would not settle in 

 the chimneys, nor would it adhere so firmly. 



A simple experiment to illustrate this tendency of dust to leave warm, and 



