Rough comparison of coefficients with Matthiessen's. It is perhaps worth while to give the following rough comparison between the results for the variation-coefficients which we have obtained in the neighbourhood of 10° C. with the mean results of Matthiessen.

There is no striking difference except in the case of Pt Ir, where the alloy of which the coil is made must approach much nearer a pure metal than Matthiessen's alloy (33.4 per cent. iridium) did.

Discrepancy in Coil G with former measurements.-The only other point to which we have to call attention is the discrepancy between former and present measurements in the coil G, whose resistance seems to have gone down since it was last tested.

In conclusion we venture to suggest two alterations in the construction of standard coils, which, as far as our experience goes, would be improvements. First, to make them flat instead of cylindrical. This would facilitate stirring when the coils are immersed in any liquid.

Secondly, to insert as near the wire as possible a properly insulated junction of a thermoelectric couple, the other junction of which should be fastened on the outer case of the coil. Several of these fitted to each coil would do away with a great deal of the trouble and uncertainty attending the temperature determinations required in comparing and copying standards.

Third Report of a Committee, consisting of Prof. A. S. HERSCHEL, B.A., F.R.A.S., and G. A. LEBOUR, F.G.S., on Experiments to determine the Thermal Conductivities of certain Rocks, showing especially the Geological Aspects of the Investigation.

THE object originally proposed by the Committee was to arrange and classify the most commonly occurring rocks experimentally according to their powers of conducting heat; and it has hitherto been so far successfully attained that the thermal conductivities of an extensive series of ordinarily occurring rocks have been shown to differ from each other on a very strongly marked scale of gradation, which it was endeavoured to represent graphically in the Committee's last Report by a series of ascending steps of absolute thermal resistance, or resistance to the passage of heat offered by the different rocks. To every 200 units of this ascending scale a new letter of the alphabet, starting with A for the interval 0-200 of absolute resistance, was assigned; the values of the resistances were shown graphically, and the various rocks that arrange themselves under the several classes so formed could be readily discerned. By adopting this graphical mode of representation the values of certain

thermal resistances observed during the past year and communicated in this Report may be exhibited with equal clearness, and an easy comparison may by this means be made of the values found in this and last year's series of experiments where the same rock-specimens, or specimens of very closely allied kinds of rock, were submitted in the former and in this year's series to examination. A slight change, however, is here introduced in briefly describing the results obtained numerically, by employing, instead of the significant figures of those results (as was done in the last Report), the tenth part of them as a brief expression for the absolute thermal conductivity. Thus the absolute thermal conductivity of galena in the present list being 0.00705 in centimetre-gramme-second units, hitherto described for brevity by its significant figures 705, will be spoken of in this Report as 70-5, to which the meaning may conveniently be attached that 70.5 gramme-degree units of heat per second pass through a plate of galena one centimetre thick, having an area of one square metre, for a temperature-difference of one degree between its faces.

The method of investigation without the use of a thermopile has hitherto proved unsuccessful, no soft material capable of effecting a close junction with the rocks having yet been found of sufficiently constant resistance to afford a useful standard of comparison with them when the rocks are introduced between its layers; but the progress of the investigation has shown that a simple water-film (if it could be preserved from drying off with porous rocks) effects a complete junction between them and any impervious surface, as that of caoutchouc, against which they are pressed. A similar film of oil, it appears from some experiments recorded in the present list, is less effective for the purpose; and to ensure a constant water-film in which the thin wires of the thermopile could be placed, pieces of well-soaked bladder kept soft in water rendered antiseptic with carbolic acid were laid on the india-rubber faces of the boiler and cooler, so as to press the thermopile-wires against the rock with a constantly moist and uniformly wet surface. The duration of an experiment and the temperature to which they were exposed (usually between 100° and 120° F.) were never so great as to cause the bladders to approach dryness before the termination of the experiment. The proportion of moisture absorbed by the rocks (when sensibly porous) was ascertained, and it was always such a small fraction of that imbibed by the same rocks thoroughly soaked in vacuo that it probably exercised a scarcely sensible influence on the results. Its amount, and that of the full quantity of water absorbable by the porous rocks tested, is stated in the list; and from the corresponding alteration of the observed conductivity some idea of the probable correction necessary to be applied for the presence of moisture in some of the porous rocks during the process of the experiment may be obtained.

The two chief defects of the thermopiles used hitherto had been their thickness (making them intrude too far from the rock-surface into the badly conducting strata with which it is in contact), and the false thermoelectric currents proceeding from irregularities of material and internal condition of the wires subjected to great varieties of temperature along their length. To diminish the former source of error, wires less than half a millimetre (0·40 millim., or inch) in diameter were used and neatly soldered at the junctions; and to counteract as far as possible the remaining evil, they were chosen of the most dissimilar metals (iron and German silver), and twelve junctions above and twelve below the rock-plate formed a continuous circuit giving a very strong thermoelectric current. The whole resistance of the circuit (including the 20 ohms usually added to bring its indications conveniently within the scale of a Thomson's reflecting galvanometer) was 40

ohms when the wires were coupled for observing a difference of temperature; and it was assumed that, with this resistance and with the probable tendency of twelve wires similarly circumstanced to neutralize each other's false effects, no sensible errors from local disturbances would arise. The instrument was submitted to some careful tests, with a result that, at the highest temperatures of the experiments, errors in the temperature-difference amounting to about 1° F. may have been committed. At the ordinary temperatures of the wires between 100° and 120° F. it was found, by substituting a heated iron disk (coated on the faces with thin paper) in the place of a rock-plate, so as to heat both sets of wires equally, that the only permanent deviations produced as the plate sunk very slowly in temperature also sunk gradually with it from an equivalent value of about 3° to about 10 upon the scale. As the correctness of the small temperature-differences (of 6° and upwards) lying usually between the above two temperatures was thus fairly checked, and for exceptionally higher differences and temperatures the conditions could not easily be more exactly assimilated to those of the actual experiments so as to control and estimate them, the effects of these small errors have not been further regarded in the calculations; but in order to avoid changes of value in the divisions of the scale, and to enable the actual temperature of each set of wire-junctions to be directly observed, an arrangement of the thermopile was made by which each set of junctions could be separately combined with a similar set in continuous circuit with the galvanometer placed in a small rectangular waterbath. The latter is made of tin, and, as well as its lid, is well jacketed with cork, and provided with an agitator; so that by adding hot or cold water, which can be withdrawn below, any temperature of the water in the bath can be obtained. A simple commutator enables the circuit with the galvanometer to be closed, either through the two principal sets of junctions or through one of them and through a set corresponding to it in the bath; so that by changing the temperature of the latter until no current passes through the circuit the actual temperature of each rock-face could be observed. This mode of observation is free from all objections, excepting those of false currents arising in long wires and plates of the same metal maintained at very various temperatures; but with the exception of the twelve loops of German-silver wire projecting on one side from the rock-plate, the corresponding loops on the other side, and all the rest of the circuits made to the galvanometer, were formed from the same piece of iron wire freshly annealed. The comb-like teeth of the commutator are pieces of narrow hoop-iron about 3 inches long, closely set together in wood, and also thoroughly annealed, to which the proper terminals of iron wire are soldered at their feet, while the upper ends are filed to chisel-edges; and a small hand-rack of iron wedges set on wood at proper distances apart, thrust between them in different positions, completes the connexion in the three different orders that are required. The additional branch wires used in the arrangement are few, and, as will be seen from the following description, add very little to the total lengths of iron wire which conduct the currents. The twelve-turn coil of wire in which the rock is pressed consists of twelve half-turns or loops of German silver and the same number of iron loops. The twelfth loop of German silver (see figure, p. 22) completes the circuit or connexion from the beginning to the end of the coil through the medium of the galvanometer. There are thus twelve junctions of dissimilar metals above, and twelve below the rockplate in a closed circuit with the galvanometer. To produce a new set of twelve junctions corresponding to each of these, the loops of German-silver wire are all cut through in the middle, and the free ends soldered to twentyfour short pieces of iron wire, the junctions being laid side by side across a

narrow water-tight trough formed of three or four rectangular washers of caoutchouc laid on a sheet-caoutchouc floor, upon which the sides of the rectangular tin bath, open at the top and bottom, are pressed down. The tin bath is 5 inches long (the same as the width of the rock-sections), nearly the same height, and 2 inches wide; and it is provided with a false bottom, through the perforations of which the water reaches the wires, and is kept agitated above by a thermometer passing through a longitudinal slit in the lid and attached to a small tin blade, without injuring them. The twenty-four extremities of iron wire projecting 1 or 2 inches beyond the bottom of the bath are there soldered to the feet of twenty-four teeth of the commutator, and the twelve iron wedges of the hand-rack being inserted between the points of these teeth, completes the circuit connexion in the ordinary way for observing a difference of temperature between the two principal sets of junctions of the thermopile. As a proof of the trustworthy action of the instrument, it may be mentioned that when, in the course of an experiment, the reading of the galvanometer with the thermopile thus joined up was being noted, and water of various temperatures from 60° F. to 160° F. was poured into the bath where the twenty-four supplementary junctions are placed and are all included in the circuit, not the smallest effect was produced upon the reading as soon as the water in the bath had by gentle agitation become uniform throughout in temperature. Not only are the two opposing sets of twelve junctions heated in the bath on the average all of exactly equal force, so as to balance each other, but the false currents, which in such ranges of temperature must be evoked with sensible intensity if any of them should prevail, either neutralize each other exactly or are entirely absent, as it appears equally probable to conjecture, in this portion of the apparatus. As regards formation of the circuit through one of the principal sets of junctions only, accompanied by a corresponding set of junctions in the bath, this is accomplished as is represented in the annexed outline sketch, where two pairs of junctions only (a b, a' b'), above and below the rockplate, are shown, thin lines representing iron and thick lines German-silver wire. B is the bath in which the supplementary junctions, s's ss', obtained by severing the loops of Germansilver wire, as at ss, are immersed. The two extreme half-loops and corresponding teeth of the commutator serve to complete the circuit with the galvanometer; and the arrangement for every additional severed loop of German-silver wire introduced between them will easily be apprehended from the single intermediate one, ss, here shown. The iron wedges, w w w, of the rack-piece pushed downwards between the yielding iron blades of the commutator are shown by black dots, forming a circuit in the usual manner for obtaining a reading of difference of temperature between the junctions a a', bb'. Each loop or half-turn (b fa, b'f'a') of iron wire is continued past

W

G

s'

SS

s'

B

23

the upper junctions (a, a') and carried through the bath to a separate tooth of the commutator; and by moving the wedges of the rack-piece together one tooth-space to the right or left (as shown in new positions by a × in the figure), combinations of junctions in the bath with junctions (a a') above or (bb) below the rock-plate are put into connexion with the galvanometer.

By the same mode of trial as before, a heated iron plate coated with thin paper being substituted in the place of an experimental plate, the temperatures of its two faces, as exhibited by the thermometer in the bath when the commutator was shifted from one of its two supplementary positions to the other, were sensibly the same as the heated plate slowly cooled, and no false difference of temperature arising from false currents differently excited in the two circuits thus joined up were found to be indicated as a result of several such determinations of the really equal temperatures of the two faces of the plate. This mode of observing the actual temperatures and the temperature-differences of the rock-faces in the present series of experiments was therefore constantly employed, and the values of the scale-divisions in degrees for the other more usual method of employing the thermopile were not determined with special care, although this adjustment of the commutator was also used to check and follow the gradual variations of temperaturedifference that were less speedily, although more certainly, measured by the absolute method of determination. The only case of failure to observe a sensible difference of temperature between the two sides of an experimental plate occurred with iron-pyrites, which (as well as galena), being a good conductor of electricity, it was found necessary to coat with two thicknesses of the thinnest tissue-paper on cach face; and the apparent difference of temperature recorded (which was decidedly less than 1°) may have arisen from the resistance offered by the slight obstructions of these thin paper sheets (soaked with water) to the passage of the heat although certainly very great, no definite value of the thermal conductivity of ordinary iron-pyrites can therefore be assigned. It was also necessary to use oil junctions instead of wet bladders, from the galvanic effects produced by the saturated salt solution, when rock-salt was tested; and it appears probable from some measurements of quartz with the same kind of luting that the conductivity of rock-salt thus found is somewhat less than, rather than likely to be in excess of, the real thermal conductivity of that substance. As a good assurance that when membranes wetted with water were used to press the thermopile against the rocks the true temperatures of their faces were very nearly marked, the experiment with iron-pyrites may be instanced, as the small temperature-difference of less than 1° could not have been observed if the wires were not very nearly indeed at the same temperature as the two papercovered faces of the pyrites against which they were pressed; and as the circumstances of their adjustment in other cases were exactly the same as in this instance, it may be assumed that the method of pressing the thermopile against the rocks with wet bladders adopted in the present series of experiments exhibited the true temperature-difference of the faces, and afforded correct values of the thermal conductivities. The pressure was applied by means of strong spiral springs (instead of the weights described in the last Report), whose extensions in a graduated tube indicated the pressures which they were made to exert. The pressure thus applied was usually 80 lbs. upon a surface of nearly 20 square inches of the rock-plates, or about 4 lbs. per square inch. The general agreement of the results with those formerly obtained also serves to verify the correctness both of the thermal conductivities now assigned and of those previously observed. The principal differences in the two methods of determination consist in the use of an im