5O * Of the Management of Fire 



interstices of those substances, and consequently in 

 impeding the passage of heat through them. 



Perhaps there may be another still more hidden cause 

 which renders one substance better than another for 

 confining heat. I have shown by a direct and unex- 

 ceptionable experiment that heat can pass through the 

 Torricellian vacuum,* though with rather more diffi- 

 culty than in air (the conducting power of air being to 

 that of a Torricellian vacuum as 1000 to 604, or as 10 to 

 6, very nearly) ; but if heat can pass where there is no 

 air, it must in that case pass by a medium more subtile 

 than air, a medium which most probably pervades all 

 solid bodies with the greatest facility, and which must 

 certainly pervade either the glass or the mercury em- 

 ployed in making a Torricellian vacuum. 



Now, if there exists a medium more subtile than air 

 by which heat may be conducted, is it not possible that 

 there may exist a certain affinity between that medium 

 and sensible bodies ? a certain attraction or cohesion, 

 by means of which bodies in general, or some kinds 

 of bodies in particular, may, somehow or other, impede 

 this medium in its operations in conducting or trans- 

 porting heat from one place to another? It appeared 

 from the result of several of my experiments, of which 

 I have given an account in detail in my paper be- 

 fore mentioned, published in the year 1786, in vol. 

 Ixxvi. of the Philosophical Transactions, that the con- 

 ducting power of a Torricellian vacuum is to that of 

 air as 604 to 1000; but I found by a subsequent ex- 

 periment (see my second Paper on Heat, published in 

 the Philosophical Transactions for the year 1792) that 



* See my Experiments on Heat, published in the Philosophical Transactions, 

 Vol. LXXVI. 



