Of Chimney Fireplaces. 535 



In like manner, when two fluids of any kind, of dif- 

 ferent densities, come into contact, or are mixed with 

 each other, that which is the lightest will be forced up- 

 wards by that which is the heaviest. 



And as heat rarefies all bodies, fluids as well as solids, 

 air as well as water or mercury, it follows that two por- 

 tions of the same fluid, at different temperatures, being 

 brought into contact with each other, that portion which 

 is the hottest, being more rarified, or specifically lighter 

 than that which is colder, must be forced upwards by this 

 last. And this is what always happens in fact. 



When hot water and cold water are mixed, the hottest 

 part of the mixture will be found to be at the surface 

 above ; and when cold air is admitted into a warmed 

 room, it will always be found to take its place at the 

 bottom of the room, the warmer air being in part ex- 

 pelled, and in part forced upwards to the top of the 

 room. 



Both air and water being transparent and colourless 

 fluids, their internal motions are not easily discovered 

 by the sight ; and when these motions are very slow, 

 they make no impression whatever on any of our senses, 

 consequently they cannot be detected by us without the 

 aid of some mechanical contrivance. But where we 

 have reason to think that those motions exist, means 

 should be sought, and may often be found, for render- 

 ing them perceptible. 



If a bottle containing hot water tinged with logwood, 

 or any other colouring drug, be immersed, with its 

 mouth open, and upwards, into a deep glass jar filled 

 with cold water, the ascent of the hot water from the 

 bottle through the mass of cold water will be perfectly 

 visible through the glass. Now, nothing can be more 



