1847.] NASMYTH ON THE SLOW TRANSMISSION OF HEAT. 233 



The instance in question, you will remember, was that of a large 

 plate-iron pot, containing eleven tons of white-hot melted cast-iron — 

 a temperature so high as to be quite beyond all thermometric certainty, 

 but well-known to be the highest intensity of furnace heat, being 

 quite equal to that of welding hot iron. 



This vast mass of white-hot melted cast-iron, you will remember, 

 stood in the pot for upwards of twenty minutes, and but for a thin 

 coating of clay and sand of about half an inch thick, would have soon 

 melted the bottom and sides of the pot. 



This half-inch thickness of mineral substance, however, was quite 

 sufficient to prevent the conduction of the heat to the exterior; so 

 completely so, that after this vast mass of hot iron had remained 

 for upwards of twenty minutes in the pot, you could place your hand 

 on the side of the vessel without feeling any inconvenient degree of 

 heat ; and, as I mentioned to you, so slowly and imperfectly does this 

 thin lining of half an inch of clay and sand permit the heat to pass 

 outwards, that the entire mass might rest there till it became cool 

 ere the outside of the pot would have reached a temperature high 

 enough to carbonize wood in contact with it, the radiation from the 

 outside carrying away the heat as fast as the slow conducting power 

 of the clay and sand lining transmits it. 



So striking an instance of the low conducting power of such sub- 

 stances is not frequently met with, and it appears to me that it is 

 calculated to remove some of the doubts occasionally expressed re- 

 specting facts which indicate a high temperature in the interior of the 

 earth. If half an inch of mineral matter thus intercepts the commu- 

 nication of so high a temperature as that of a mass of eleven tons of 

 white-hot cast-iron, what may not two or three hundred miles of 

 similar substances effect, in preventing the central heat of the earth 

 from developing its action beyond a very moderate extent towards the 

 surface? If this reasoning be correct, it tends to show that there 

 may exist below the crust of the earth, a mass of fluid molten matter 

 which at depths of two or three hundred miles may have a tempera- 

 ture transcending all our ideas of high heat, the only indications of 

 which, at the surface, are afforded by volcanos, hot springs, and that 

 regular increase of temperature as we descend towards the interior 

 found in deep mines, or by deep borings. 



There are many other instances of this nature which I could bring 

 forward, as exhibiting the remarkable non-conducting powers of clay ; 

 many such examples are every day before the eyes of our manufac- 

 turers who have to do with furnaces where intense heat is employed. 

 The fire-brick lining of such furnaces is only from A\ to 9 inches 

 thick, and yet while the heat within is as high as our furnace powers 

 will carry it, the hand can be placed outside without suffering any 

 inconvenience. 



