Academy of Science, 49 



INTERNAL HEAT OF THE EARTH. 



By Prof. B. F. Mudge. 



We frequently notice, in newspapers and popular treatises on geology, the theory 

 stated, in the most deliberate manner, as if it were accepted by all scientific men, 

 that heat rapidly and regularly increases from the surface to the center of the earth. 

 They tell us that at the depth of twenty or thirty miles, and from thence to the 

 center, the whole mass of the globe is in a fluid, molten state, having an intensity 

 of heat which even the imagination of the chemist can hardly comprehend. This 

 theory is based on the hypothesis that, starting from the surface, there is a con- 

 stant and uniform increase of heat, at the rate of one degree F. for every forty-five 

 feet in depth. This would give a temperature of 212", or that of boiling water, at 

 little more than a mile. As iron melts at 3,000°, that temperature would be reached 

 at twenty-five miles. At a depth of about fifty miles a heat would be found, suffi- 

 cient to melt the most infusible rocks. Most of our strata, like limestone, would 

 yield at a less heat. At the center of the earth, by the popular ratio, it would be 

 600,000", or eighty-five times greater than tliat required to melt any combination of 

 natural elements (except platinum) found in the earth's crust. Some observations 

 make an increase of heat equal to one degree for every thirty-two feet. Thiswould 

 place the molten ocean nearer the surface of the earth, the crust thinner, and the 

 heat at the center more intense. 



This theory was advanced by some of the early writers, on a limited array of 

 facts, and a narrow comprehension of nature's laws. We know of no geologist of 

 high standing at the present day who sustains this theory. 



The first and great objection to this hypothesis lies in its opposition to well-known 

 chemical laws. Every chemist who fuses metals in the laboratory, and every fur- 

 nace-man in our iron, zinc or copper foundries, knows that when he has raised his 

 crucible, or furnace, to the melting point of any metal, he can by adding fuel raise 

 the molten material to far higher degrees of heat, but if he throws in pieces of the 

 unmelted metal the temperature will not rise beyond the melting point, till the last 

 ounce has become liquid. Thus, lead will melt at 625°, but after melting, its tem- 

 perature can be raised hundreds of degrees till it boils, or even becomes vapor. 

 But if when the lead has just reached the nulting point, bars of lead, or even shot, 

 are added to the liquid surface of the furnace, the temperature will be constant at 

 G2~)°. This rule applies to all known substances. It is often seen in the familiar 

 example of water. So long as a fragment of ice remains in our kettles, no fire will 

 raise the temperature of that fluid above 32°. Apply this simple chemical test, and 

 it at once settles this question, that the interior of the earth, if liquici, can not be 

 above the melting point (7,000°) of the most refractory portions of the earth's 

 crust. If the heat of the interior exceeded that point, this thin crust, whether of 

 twenty-five or one hundred miles, would be immediately melted into the common 

 mass, and we should not be here to-day to discuss its plausibility. 



Still, it remains to consider how far we have any data to fix the rate of increase 

 of beat, and at what depth the crust of the earth may be in a melted state. Let lis 

 k)ok at the facts on which the theory of a regular and uniform increase of heat 

 downward, is based. 



Prof. Win. B. Rogers, on examiuation of the coal mines of Eastern Virginia, 

 computed ttie increase at one degree for every sixty feet. In the United Copper 

 4 



