272 SCIENTIFIC AMUSEMENTS. 



of conveying heat from one spot to another by causing a heated 

 mobile fluid to move as a whole is termed convection, and is 

 applicable to liquids as well as to solids. On the other hand, if 

 a poker is put in the fire, by and bye its other end becomes more 

 or less warm. The process by which the heat travels along the 

 length of the poker from fire to handle end is termed conduction. 

 Yet another way in which heat is propagated from spot to spot is 

 by the process termed radiation, best exemplified by the heat of 

 the sun, or by the effect produced by holding a red hot poker a 

 foot or so from the face and level with it, so that no currents of 

 hot air can reach the face by convection ; the sensation of warmth 

 on the skin and the effect produced on the eye by the emission 

 of light are usually spoken of as manifestations of radiant energy. 

 Some sources of radiant energy are also capable of setting up 

 chemical action ; as, for example, the effect of light in causing the 

 combination of chlorine and hydrogen gases (Expt. 215), or in 

 decomposing certain silver compounds (Expt. 196). 



Expt. 320. Convection Currents in Water and Gases. 

 Obtain a large glass shade (or a flask or bolt-head) nearly full of 

 water, and set it over a large gas burner or spirit lamp, so as to 

 heat it up gradually ; throw into the flask some bran or finely 

 chopped up wooden shavings, when the formation of currents of 

 heated water rising from the bottom to the top and of descending 

 currents of cooler water will become visible by the motion of the 

 particles of bran. Another way of rendering the currents visible 

 is to throw into the flasks some lumps of solid litmus, fragments 

 of the "lead" from an aniline dye pencil, or other material that 

 will tint the water without dissolving too quickly ; the flow of 

 differently tinted streams will become easily manifest. 



The ordinary processes in use for heating houses, churches, &c., 

 by hot water pipes is based on this property of convection in 

 liquids. A boiler or other heating arrangement with a fire attached 

 is fixed in the vault or basement, and connected with the supply 

 pipe for hot water at the top of the boiler ; the return pipe, bring- 

 ing back the cooler water after it has done its work and warme d 

 up the air in contact with the coils of piping arranged for the 

 purpose, joins the boiler at the bottom ; this construction evidently 

 necessitates that, as the water in the boiler becomes heated, it will 

 rise upwards and flow through the supply pipe, its place being 

 taken by the cooler fluid entering by the return pipe. 



Instead of plain water, solutions of various saline matters are 

 sometimes employed, these being capable of being more highly 

 heated than water only. 



When it is required to cool the air of a building, an analogous 



