Lesson VII.; 



CHEMISTRY. 



113 



by heat. We have already learned (p. 22) how heat is communicated, and it has 

 been found by experiment that certain bodies are better conductors than others, 

 ^ s^ hence they have been divided into coi</MC/ors and non~conductort, and 

 their relative conducting power has been given by Despretz as follows : 



Tin 303.9 



Lead 



Marble 23.6 



Porcel.in 1 J J 



Fine Clay 11.4 



Gold 1000 



- r - Copper 898.2 



fedl &* I'latinum 381 



Iron 374.3 



Zinc 363 



From this we learn that metals are the best conductors ; liquids and 

 aeriform fluids are very imperfect conductors of caloric. When a solid 

 becomes a liquid, a certain amount of heat becomes latent, that is to say, 

 it is, as it were, squeezed out and disappears ; and when a liquid becomes 

 a solid, a corresponding effect is produced. Intent heat varies according 

 to the substances operated on ; for example, the heat of fluidity in 



Bismuth 550 



Tin 500 



Zinc 493 



Bees- wax 175* 



Lead 162" 



Spermaceti 145 



Sulphur 143 



/if.u. Water 142* 



All bodies expand by heat, and contract on cooling. 



QUESTIONS. 



H H 



r. "When we place a vessel con- 

 taining water upon the fire, how is the 

 heat diffused ? 



/. Hy confection that is, the heated 

 particles ascend, or arc 

 carruil to the top of the 

 vessel. 



/'.Why does the 

 water roll about and bub- 

 ble up when it boils ? 



P. Ik-cause the par- 

 ticles of which it is composed, are con- 

 tantly put in motion by the contending 

 >, and the escape of steam. 



/'. What currents arc they of which 



ak? 



An atcending hot current in tin- 

 centre of the von*- / /. 14), and a 

 detetnding cold curr f ,,t on each side of the 

 TWM! (c c, Fig. l 



/'. When we wish to measure high 

 degrees of heat in chemical exper. 

 how do we manage ? 



P. If the temperature is very high, like 

 that of furnaces, we use one of 

 Daniell's platina pyrometers, but if 

 only for boiling liquids, we use a 

 cylindrical thermometer, graduated 

 on the glass tube to 572 3 Fal : 

 which may be fitted into a cork, and 

 I adapted for any \essel, or 

 allowed to float in the liquid. 



/.What are the chief effects 

 produced by heat ? 



/'.Expansion, liquefaction, v.- 

 porisation, evaporation, and bun. 

 or ignition. 



77. 7'. How do you know 

 expansion is caused by heat ? 



P. By experiment and obst :, 

 The iron rim of a cart wheel is ex; 

 by heat before it is put on, nn<l tin 

 water is thrown over it, it cools ai 

 tracts, thus binding the spokes and c 

 together. 



[fee Question H, P. 8; and Question it and 

 , p. 13.] 



ft 



