Spo Gi/UTnl Danriru cf IL.it. 



in tiicir fcnfible heat. Tiicfe important rcfults conflitutc tliC foundation of the theory of 

 the ilitievcat capacities of bodies for heat, or their fpecific heats when at the fame tem- 

 perature. 



Another difcovery, of no lefs magnitude, feemed to render us acquainted with the prin- 

 cipal caufes by which the mutations of temperature are originally eBciTlcd. It was found 

 by experiment, not only that different bodies require greater or Icfs quantities of heat to be 

 communicated in order to rnifc their temperature through equal numbers of degrees, but 

 alfo that the capacity of any individual fubftance is lead when in the folid (late, greater in 

 the fluid ftate, and greatcft of all when converted into elaftic vapour or air. So that, as a 

 natural confequence, a much greater portion of heat will be required to convert ice into 

 water, and rnife its temperature ten degrees, than would Iiave been neccflary to have equally 

 elevated the temperature of cold ice without melting it. And fo lilcewife the vaporiza- 

 tion of water, without raifing its temperature beyond the boiling-water point, requires a 

 much larger quantity of heat than would have heated the fame raafs of water from the 

 freezing to the boiling point. 



A great number of iiiterefting dedu£tions, from thefc fa£ls, have-been applied to explain 

 the general mutations of heat in the univerfe around us. for, in the firft place, fince 

 every change of chemical combination is attended with a greater or Icfs change of the ca- 

 pacities of the aggregate, it almoft invariably happens that the temperature of the new- 

 compound is either above or below the common temperature of the atmofphere ; and as the 

 fufibilities of thefe compounds are alfo afFedled according to laws of which at prefent we 

 know nothing, there is very frequently a change of temperature on this account alfo. 



Such is the mod luminous and beautiful theory of heat which our cotemporaries have 

 begun to develop, and which is at prefent explained at confidernble length in moft ele- 

 mentary works. But as this fubjeft appears to depend upon fadls fcarcely capable of being 

 extended farther by hypothefis or analogical reafoning, it has happened that induftions 

 experiments and applications, though of the greateft value, have been but flowly made. The 

 ciTeft of chemical operations on the temperature of bodies has been applied with gi'eat fuc- 

 tcfs to explain the adl of combuftion ; and the general confequences of the change of 

 capacity in the folid, fluid, and vaporous ftates have been confidered with regard to their 

 extenfivc influence upon the face of the globe. But thofe refults which originate chiefly 

 from the dilicrences of condutHng power hi bodies with regard to heat, have fcarcely con- 

 ftitutcd an object of direft enquiry among philofophers, excepting by Count Rumford in 

 the Philofophical Tranfaftions, and in his Experimental Eflays, at prefent in the progrcfs of 

 publication. On the prefent occafion I fliall not enter into the confideration of the extenfive 

 economical ufes to which this philofopher has applied the refults of his experiments and 

 deductions, but fhall chiefly confine the prefent memoir to the confequences which he has 

 fliewn to arife from the imperfedt conducing power of fluids with regard to heat. 



The free paflagc of heat, in all direftions, through all kinds of bodies, has never yet been 

 called in quedion, though the rapidity of its tranfmiffion is well known to difl^er exceedingly 

 in various kinds of bodies. Under the influence of this opinion Count Rumford began his 

 experiments on heat. His former experiments (hewed that air is a non-condusSor of heat ; 

 and the late experiments contained in his feventh Efliiy afccrtain that water is in the 

 fame predicament. He thinks that all other fluids have the fame property. He was led 



