270 ANNUAL OF SCIENTIFIC DISCOVERY. 



imcnts on soft substances, such as spermaceti, wax, and sulphur, that 

 the temperature of fusion increases about 1.3 Fahrenheit for every 

 500 pounds' pressure per square inch, that is, in other words, that 

 the temperature of fusion under pressure is increased in that ratio. 

 If we assume this to be the law for the materials of the earth's crust, 

 and correct our previous calculations in accordance with it, we shall 

 find that we shall have to go to a depth of sixty-five miles, instead 

 of merely forty, before the point of fusion of the rocks is reached. It 

 must, however, be observed that Mr. Hopkius's later experiments 

 with tin and barytes do not show such an increase of the point of 

 fusion in consequence of pressure, and he is led to the belief that it 

 is only in the more compressible substances that the law holds true. 

 Independently of this, however, Mr. Hopkins points out to me that in 

 the above calculation it is assumed that the conductivity of the rocks 

 is the same at great depths as at the surface. In opposition to this, 

 he has shown experimentally that the conducting power for heat is 

 at least twice as great for the dense igneous rocks as for the more 

 superficial sedimentary formations of clay, sand, chalk, etc. And 

 these close-grained igneous rocks art those which we believe must 

 most resemble the rocks at great depths below the surface. Now, 

 Mr. Hopkins shows that if the conductive power were doubled, the 

 increase of depth, corresponding to a given increase of temperature, 

 would be doubled, and we should probably have to descend eighty or 

 a hundred miles to reach a temperature of 3,000, besides the further 

 increase which investigation in.iy show to be due to the influence of 

 pressure on the temperature of fusion. Mr. Hopkins therefore con- 

 cludes that the extreme thinness of the crust assumed by some geolo- 

 gists to account for volcanic phenomena is untenable. Calculations 

 on entirely independent data led him to conclude that the thickness 

 did not fall short of eight hundred, instead of thirty or forty, miles. 

 If it be so much, he is further led to believe that the superficial tem- 

 perature of the crust is due to some other cause than an internal 

 fluid nucleus. It remains a problem, therefore, which my friend Mr. 

 Hopkins is endeavoring to solve, as to what is the actual condition 

 of the earth at great depths, and the relation of terrestrial heat to 

 volcanic phenomena. 



Mr. Hopkins said the communication which had been made by the 

 President of the Association was worthy of more confidence than any 

 which had hitherto been given to the public. No former treatises 

 on this point had so largely taken into account the various circum- 

 stances in connection with mines, and the causes incident to them, 

 which affected the temperature of the earth's crust. The condition 

 of the rocks and walls, as well as the water in mines, must necessarily 

 have a varying effect upon the temperature, and these facts had not 

 received sufficient attention at the hands of those who had made ex- 

 periments, but the President had now gone largely into it. One 

 great advantage likely to result from the experiments explained in 

 the President's paper was that the experiments had been made in a 

 virgin mine, before it had been worked, and the temperature ascer- 

 tained before being altered by working. Now, as regarded the mine 

 in question, the stratum was very much inclined, and there was a 

 good deal of water in it. That being the case, great caution was re- 



