June 20, 1895] 



NATURE 



183 



Ijy another equal quantity of cold water. The chief difficulty 

 in respect to steadiness of temperature was the keeping of the 

 i;as lamp below the bath of melted tin uniform. If more 

 experiments are to be made on the same plan, whether for rocks 

 -or metals, or other solids, it will, no doubt, be advisable to use 

 an automatically regulated gas flame, keeping the temperature 

 cif the hot bath in which the lower face of the slab or column is 

 iuimersed at as nearly constant a temperature as possible, and 

 ti) arrange for a perfectlj' steady flow of cold water to carry away 

 heat from the upper surface of the mercury resting on the upper 

 side of the slab or column. It will also be advisable to avoid 

 the complication of having the slab or column in two parts, 

 when the material and the dimensions of the solid allow fine 

 perforations to be bored through it, instead of the grooves 

 which we found more readily made with the appliances avail- 

 able to us. 



§ 4. Our first experiments were made on the slate slab, 

 25 cm. square and 5 cm. thick, in two halves, pressed together, 

 each 25 cm. by I2"5, and 5 cm. thick. One of these parts 

 cracked with a loud noise in an early experiment, with the lower 

 face of the composite square resting on an iron plate heated by 

 a powerful gas burner, and the upper face kept cool by ice in a 

 metal vessel resting upon it. The experiment indicated, very 

 decidedly, less conductivity in the hotter part below the middle 

 than in the cooler part above the middle of the composite 

 square slab. We supposed this might possibly be due to the 

 crack, which we found to be horizontal and below the middle, 

 and to be complete across the whole area 

 ■of 12^ cm. by 5, across which the heat 

 ■was conducted in that part of the com- 

 posite slab, and to give rise to palpably- 

 imperfect fitting together of the solid above 

 and below it. We therefore repeated the 

 experiment with the composite slab turned 

 upside down, so as to bring the crack in 

 one half of it now to be above the middle, 

 instead of below the middle, as at first. 

 We still found, for the composite slab, 

 less conductivity in the hot part below the 

 middle than in the cool part above the 

 middle. We inferred that, in respect to 

 thermal conduction through slate across 

 the natural cleavage planes, the thermal 

 conductivity diminishes with increase of 

 temperature. 



§ 5. We next tried a composite square 

 slab of .sandstone of the same dimensions 

 as the slate, and we found for it also decisive 

 proof of diminution of thermal conductivity 

 ■with increa.se of temperature. We were 

 not troubled by any cracking of the sand- 

 stone, with its upper side kept cool by 

 an ice-cold metal plate resting on it, and 

 its lower side heated to probably as much as 

 300' or 400^ C. 



§ 6. After that we made a composite 

 piece, of two small slate columns, each 

 3-5 cm. square and 6-2 cm. high, with 

 natural cleavage planes vertical, pressed 

 together with thermoelectric junctions as 

 before ; but with appliances (see § 10) for 

 preventing loss or gain of heat across the vertical sides, which 

 the smaller horizontal dimensions (7 cm., 3-5 cm.) might require, 

 but which were manifestly unnecessary- with the larger horizontal 

 ilimensions (25 cm., 25 cm.) of the slabs of slate and sandstone 

 used in our former experiments. The thermal flux lines in the 

 former experiments on slate were perpendicular to the natural 

 cleavage planes, but now, with the thermal flux lines parallel to 

 the cleaviige planes, we still find the same result, smaller thermal 

 conductivity at the higher temperatures. Numerical results will 

 be .stated in § 12 below. 



§ 7. Our last ex|)eriments were made on a composite piece 

 of Aberdeen granite, made up of two columns, each 6 cm. high 

 and 7-6 cm. square, pressed together, with appliances similar to 

 those described in § 6 ; and. as in all our previous experiments 

 on slaleand sandstone, we found less thermal conductivity at higher 

 temperatures. The numerical results are given in § 12. 



§ 8. The accompanying diagram ( Fig. i ) represents the ther- 

 mal appliances and thermoelectric arrangement of §!; 6, 7. The 

 columns of slate or granite were placed on supports in a bath of 

 melted tin with about 0'2 cm. of their lower ends immersed. 



The top of each column was kept cool by mercury, and water 

 changed once a minute, as described in § 3 above, contained in 

 a tank having the top of the stone column for its bottom, and 

 completed by four vertical metal walls fitted into grooves in 

 the stone, and made tight against wet mercury by marine glue. 



§ 9. The temperatures S'(B), <'(M), vCY) of B, M, T, the hot, 

 intermediate, and cool points in the stone, were determined by 

 equalising to them successively the temperature of the mercury 

 thermometer placed in the oil-tank, by aid of thermoelectric cir- 

 cuits and a galvanometer used to test e(|uality of temjjerature by 

 nullity of current through its coil when placed in the proper 

 circuit, all as shown in the diagram. The steadiness of tempera- 

 ture in the stone was tested by keeping the temperature of the 

 thermometer constant, and observing the galvanometer reading 

 for current when the junction in the oil-tank and one or other of 

 the three junctions in the stone were placed in circuit. We also 

 helped ourselves to attaining constancy of temperature in the 

 stone by observing the current through the galvanometer, due to 

 differences of temperature between any two of the three junctions 

 B, M, T placed in circuit with it. 



§ 10. We made many experiments to test what appliances 

 might be necessary to secure against gain or loss of heat by the 

 stone across its vertical faces, and found that kieselguhr, loosely 

 packed round the columns and contained by a metal case sur- 

 rounding them at a distance of 2 cm. or 3 cm., prevented an)^ 

 appreciable disturbance due to this cause. This allowed us to 

 feel sure that the thermal flux lines through the stone were very 



Fig. I. — Iron wires.-ire marked /. Platinoid wires are marked/. B. M, T. Thermoelectric junctions in 

 slab.. X. Thermoelectric junctions in oil bath. A. Bath of molten tin. C. Tank of cold water. D. 

 Oil bath. E. Thermometer. F. Junctions of platinoid and copper wires. The wires are 

 insulated from one another, and wrapped altogether in cotton wool at this part, to secure equality 

 of temperature between these four junctions, in order that the current through the galvanometer 

 shall depend solely on differences of temperature between whatever two of the four junctions. 

 X, T, M, B, is put in circuit with the galvanometer. G. Galvanometer. H. Four mercurj-cups. 

 for convenience in connecting the galvanometer to any pair of thermoelectric junctions, x, b, fft, t, 

 are connected, through copper and platinoid, with X, B, M, T, respectively. 



approximately parallel straight lines on all sides of the centra 

 line BMT. 



§ II. The thermometer which we used was one of Casella's 

 (No. 64,168) with Kew certificate (No. 48,471) for temperature 

 from o^ to 100^, and for equality in volume of the divisions above 

 100°. We standardised it by comjiarison with the constant 

 volume air thermometer' of Dr. Bottomley with the following 

 result. This is satisfactory as showing that when the zero error 

 is corrected the greatest error of the mercury thermometer, 

 which is at 211° C, is only 0-3". 

 Reading. 



NO. 1338, VOL. 52] 



Air 



thermometer. 



O 

 120-2 ... 

 166-8 ... 

 2II-I ... 

 2657 - 

 ' Phil. Mag,, .\ugust i3 



Mercury 

 thermometer. 



Correction to be subtracted 

 from reading of mercurj- 

 thermometer. 



1-9 I'Q 



122-2 2-0 



16S-6 1-8 



212-7 1-6 . 



267-5 '"8 



, and Roy. Soc. Edin. Prsc. January 6, 



