Conductivities of Cements and other Substances. 499 



which the upper plate rested ; steam was passed through the 

 box at the top, and the temperature of the upper plate raised to 

 nearly 100° C. The lower plate was next heated a few degrees 

 above 100° 0. by means of a gas-flame and then allowed to 

 cool, and as soon as its temperature fell to 100° C. the supply 

 of steam to the upper can was cut off and the hot water in 

 the can allowed to run out. 



Both plates were then found to cool at almost the same 

 rate ; the difference in the temperatures of the two plates never 

 exceeded 1° C., the upper plate being always the warmer. 

 Since the conductivity of air is only about '00005 C.Gr.S. 

 units, the amount of heat conducted from the upper plate to 

 the lower is negligible, so that each plate cools independently 

 of the other. 



The thermometer in the lower plate was watched as the 

 temperature fell, and the times at which the mercury passed 

 every alternate degree were observed ; the results are given 

 in the table below. 



The temperature of the air was steady at 18° C. 



Temperature of 

 lower plate. 



Time. 



Fall of 



temp, per 



second. 



Loss of 

 heat, gram- 

 degrees per 



second. 



Loss of 

 heat per sq. 

 cm., gram- 

 degrees per 



second. 



h v 



»i- 



O-0 O - 



°c. 



°C. 



h m s 











96 



78 



3.17.42 











94 



76 



18.37 



'0370 



392 



•0255 



•000336 



92 



74 



19.32 



354 



75 



43 



28 



90 



72 



20.30 



340 



60 



34 



25 



88 



70 



21.30 



325 



44 



23 



19 



86 



68 



22.33 



308 



26 



12 



12 



84 



66 



23.40 



301 



14 



04 



09 



82 



64 



24.46 



290 



07 



•0199 



11 



80 



62 



25.58 



274 



2-90 



86 



03 



78 



60 



27.12 



263 



79 



81 



02 



76 



58 



28.30 



256 



71 



76 



03 



74 



56 



29.48 



250 



65 



72 



07 



72 



54 



31.10 



244 



56 



68 



11 



70 



52 



32.32 



235 



49 



62 



12 



68 



50 



34.0 



220 



33 



51 



02 



66 



48 



35.34 



208 



20 



43 



•000298 



64 



46 



27.12 



197 



08 



35 



93 



62 



44 



38.57 



187 



1-98 



29 



93 



60 



42 



40.46 



180 



94 



24 



95 



58 



40 



42.40 



169 



79 



16 



90 



56 



38 



44.42 



156 



65 



07 



82 



54 



36 



46.57 



146 



55 



•0101 



81 



52 



34 



49.15 



140 



48 



•0096 



82 



50 



32 



51.43 











2M2 



