Then, again, they are quarreling about the leading principles of the science. Hopkins attempts to prove that the crust of the earth is eight hundred miles thick, while Humboldt asserts that it is less than twenty-four. As the temperature increases one degree for every forty-five feet we descend into the earth, so, at that rate, in less than twenty-four miles the heat would be so great as to melt iron and almost any known substance. But here, again, they differ. Wedgewood declares that iron melts at 21,000 degrees Fahrenheit; while Professor Daniels is positive that it melts at 2,786 degrees, Fahrenheit. Only a slight difference of 18,214 degrees. But then comes the great question: if granite is the lowest layer in the strata, what is below the {48} granite? De Beaumont affirms that "the whole globe, with the exception of a thin envelope--much thinner in proportion than the shell of an egg--is a melted mass kept fluid by heat, but constantly cooling and contracting its dimensions and occasionally cracking and falling in, and squeezing upwards large portions of the mass, thus producing those folds or wrinkles which we call mountain chains." On the other hand, Davy and Lyell think that "we may perhaps refer the heat of the interior to chemical changes going on in the earth's crust." So much for the uncertainties of geology. If space permitted, it would be easy to go over other sciences and show similar uncertainties in them all. It is worthy of notice that the uncertainties of science increase just in proportion to our interest in it. About what does not concern us, it is very positive; but very uncertain about our dearest interests. The astronomer may calculate with considerable certainty the movements of distant planets with which we have no intercourse; but he cannot predict the heat or cold, clouds or sunshine, and other phenomena continually occurring on our earth. The forces of heat may be measured, to some extent, but what physician can measure the strength of the malignant fever that is destroying the life of his patient. The chemist can thoroughly analyze any foreign substance, but the disease of his own body, which is bringing him to the grave, he can neither weigh, measure nor remove. Science is very positive about distant {49} stars and remote ages, but stammers and hesitates about the very lives of its professors.