Sept. 9, 1880] 



NATURE 



439 



thermometers shows us the degree o temperature of 

 the surrounding air. Again, the heat mparted to the 

 air within a paper fire-balloon makes it expand and 

 become specifically lighter than the surrounding atmo- 

 sphere through which it rises. In general it may be 

 asserted that matter, in whichever state it may be — solid, 

 liquid, or gaseous — expands when heat is imparted to it, 

 and contracts when heat is taken from it. Fig. 1 1 illus- 



trates a very simple manner of showing the expansion of 

 air when heated. An empty wine-bottle is placed with 

 its mouth downwards in a deep dish or jar containing 

 water, the bottom of the bottle projecting over the side 

 of the jar. Heat is then applied by means of a spirit- 

 lamp ; or, if this is not available, by burning under it a 

 piece of cotton wool soaked in spirits and held on the 

 end of a fork. The glass of the bottle becomes hot — if 



too hot it may crack— and the air inside shares its warmth 

 and begins to expand. There being only a limited space 

 inside the bottle, some of the air will be forced out, and 

 will rise in bubbles through the water. If now the flame 

 be removed, the reverse operation of contraction by 

 cooling may be witnessed, for as the air inside the bottle 

 cools it will occupy a smaller and smaller amount of 

 space, and the water will gradually rise up in the bottle- 



neck. Of course this is seen better with a bottle of clear 

 glass than with one of a dark or opaque tint. 



The contraction of a liquid on cooling can be even 

 more simply shown. Take a common medicine bottle. 

 Warm it gently (by rinsing it out with a little hot water) 

 so that it shall not crack by the sudden heating, and then 

 fill it brimful of boiling water. Leave it to cool ; and in 

 less than half an hour you will find that the water which 

 you poured in to overflowing has shrunk down into the 

 neck of the bottle, having contracted as it cools. 



It was mentioned above that the hot air in a fire-balloon 

 raises it, being lighter than the cold air. In the same 

 way hot water will rise through cold, and float on the top 

 of it, being specifically lighter. You may prove this in 

 several ways. Fill a deep jar with water, and then, 

 taking a red-hot poker, plunge about an inch of the tip 

 of it into the surface of the water. Presently the whole 

 of the water at the top will be boiling furiously ; but the 

 water at the bottom will be just as cool as before, for the 

 hotter water will not have gore down, but will have 



floated at the top, being lighter in consequence of expan- 

 sion. The same thing can be shown very prettily by the 

 following simple experiment. Fill a wide and deep glass 

 jar — the glass of a parlour-aquarium will do excellently — 

 to about half its depth with cold water. Provide yourself 

 also with a kettle full of boiling water, a funnel, a bit of 

 Avood about three inches square, and with some ink — red 

 ink if possible. Pour into the kettle enough of the ink to 

 colour it with a perceptible tint ; this is simply that you 

 may be able to distinguish between the colourless cold 

 water and the coloured hot water which you are going to 

 cause to float at the top. The only difficulty of the 

 experiment is how to pour out the hot water without 

 letting it mix with the cold water. Fig. 12 shows how 

 you may do this with the help of the things you have got 

 together. The bit of wood 'or cardboard) is laid on the 

 water as a float, and you must pour the hot water on to 

 this to break the force of its fall. The funnel will also 

 help to break the fall of the hot water, and will aid you to 

 guide the stream on to the middle of the float. With 



