In the paper whose title is given above (Philosophical Transactions, part i. 1873) the author has shown upon experimental data, and upon the acknowledged basis that the amount of heat annually dissipated from our globe equals that evolved by 777 cubic miles of ice at 32° melted to water at the same temperature, what is the amount of heat that can be annually produced by the transformation of the mechanical work of mean rock when crushed by the descent of the external shell upon the nucleus contracting beneath it ; he has also estimated the annual supply of heat necessary for the maintenance of the volcanic activity at present existing upon our globe ; has shown that its total amount cannot exceed a small fraction of the entire heat dissipated annually, being only 1/1589 thereof, or, in terms of crushed mean rock, equal 0.5579 of a cubic mile (paragraphs 179 and 197) ; he has also given, in Table II. (page 201) and succeeding paragraphs, his experimental results as to the contraction by diminution of temperature of melted matter that may be presumed similar to the rocky material of our globe from which natural lavas are derived. This contraction in volume, in relation to temperature between that of the blast-furnace and of the atmosphere, is shown graphically by the curve Plate x. of the above paper, the upper and lower portions of the curve being derived from experiment. The preceding elements afford some of the data necessary for any calculation as to the actual contraction of our globe now taking place annually by its secular refrigeration; but the author refrained from attempting any such calculation on the grounds that other data indispensable to any certain result are yet wanting. If we knew the thickness of the earth’s solid shell and the true increment of hypogeal temperature from the surface to the centre, or even the mean temperature of the nucleus and the nature of the whole of the matter composing the latter, we might with some assurance approximate to the amount of annual contraction of the globe due to refrigeration. But of the deep interior of our planet we really
know
but two things, viz. that the interior is hotter than the exterior, and what is the mean density of the whole. By making certain suppositions, however, as to some of the chief data wanted, we may approximate to some probable measures of the present annual contraction, and be enabled to see how far the results tend to sustain or overthrow the views enunciated by the author as to the nature and origin of volcanic heat and energy, and may also find that they throw some additional light upon the conjectured thicknesses that have been assigned to the earth’s solid crust, as well as upon the question left undecided by Laplace as to how far the effects of contraction due to refrigeration would be astronomically observable during the period of scientific history. In the author’s paper above referred to he has only dealt with the total contraction of the slag experimented upon between the temperature of its issue from the blast-furnace (viz. 3680°) and that of the atmosphere (53°), or by volume from 1000 to 933 for 3617° Fahr., from which the Rev. O. Fisher has calculated a mean coefficient of contraction =0-0000217 for 1°Fahr. (Geol. Mag., February 1874). This, though sufficient for that able writer’s immediate object, is not quite correct, as it treats the curve of contraction (Plate x. Philosophical Transactions, 1873) as a straight line. And in order to make use for our present purpose of these experimental contractions, it is necessary to obtain partial mean coefficients for different portions of the entire curve. This the author has done for ranges of about 500° between the temperatures of the blast-furnace and that of the atmosphere.
Preparation and tensile properties of guar gum hydrogel films
