84? ON THE POWER O* FLUIDS TO CONDUCT HEAT* 



In thefe experiments we may confider mercury and air 

 mix*d together of unequal temperatures, with a thin partition 

 of glafs — and from the laft we may conclude, that the thermo- 

 meter imparted to the air 40 times more heat in half a minute, 

 when its temperature was 30° above the air, than when it 

 was only ]° above it. 

 Argument^ from We fhall now advert a little to Count Rumford's experi- 

 experiments. ments. It will eafily appear, that arguing fairly upon his own 

 No water but hypothecs he can never account for the phenomena obferved : 

 cou'lddeftend ' *° r ' * lot water De »"g poured upon ice, an internal motion 

 to the ice, and would take place near the furface of the ice, by which a ftra- 



confequentlythis tum f water f a certain thicknefs would be reduced to 32°, 



only would be 



cooled, and the and then all further reduction of the ice mull: ceafe ; becaufe 



fufion of the ice a \\ the fuperincumbent water being above 53° would be lighter 



would then , ,"..,- - , . ° „ , . . . 



cea f e% con- aml coui( l not delcend to the ice. But this is quite contrary 



trarytofaft. to what took place. The facts, however, will admit of a 



fatisfactory explanation upon efiablimed principles. 

 Explanation By experiments 10 and 11, it appears, that the quantity of 



faSs. The ice ^ eat g* ven out by a body, during any fmall given portion of 

 is fufed (i) by time, is nearly as the excefs of the temperature of the body 

 dua^power" aDove tJie cooling medium. Hence, then, we may conclude, 

 of the water ; that the effect of hot water upon ice arillng from the proper 

 and (2) by its conducting power of water, will be nearly as the heat of the 

 firft is as the water. What effect the other caufe may produce, it will be 

 heat, and the difficult to determine from theory : experience will be the 



fecond dimi- i n , . .'-*•„ ' , , , 



nifhes in forne De " guide. One thing, however, appears pretty certain, 

 unknown ratio that its effect mull be a maximum, when the temperature of 

 differs from 61 "' *^ c water at large is 42f° ; becaufe then there can never want 

 42^.° a determination of the particles doivnuard to fupply the place 



of the lighter water of 32° afcending. If the temperature of 

 the water exceed 42| °, then the effect of the internal motion 

 Water at 421° will be lefs, diminifliing by fome unknown ratio. As far as I 

 f chTeV b° its 6 can J uc ^S e ^ rom Count R's experiments, the joint effects of 

 motion) nearly thofe two caufes ihould be nearly the fame with water of 42* 

 the fame as wa- an( j water f 190 o e Taking this, therefore, for granted, we 

 (chiefly by its ma ^ De enabled to fketch a table of the values of thefe two 

 heat.) caufes for every 10° of temperature. The numbers exprefilng 



the effect of the proper conducting power, are derived from 

 the 10th experiment, and coniequently are not purely hypo- 

 thetical: thofe exprefling the other effect, except 42° and 

 192°, are put down hypothetically, becaufe the law of de- 

 1 creafe has not been ascertained. 



4 It 



