In Table II. L is the heat of evaporation; t the temperature of the boiling-point; M the molecular weight; ML T the quotient of the molecular heat of evaporation by the absolute temperature: according to Tronton's Law this should be constant. If the heat of vaporisation of any one liquid be known, the absolute value of the heat of vaporisation for any other liquid can be calculated from the ratio. Water was originally selected as the standard liquid, but it proved to be quite unsuitable experiments in which one of the liquids was water never gave concordant results. It appears impossible by any ordinary means to get water so pure that conduction across it and the consequent polarisation may be disregarded. The liquid finally adopted as the standard is benzene, for which L= 944 at 80°-2.1 The heat of evaporation may also be calculated from the thermodynamic dp. T equation, L = (s′ — 8) when the necessary data can be found. dt J' 2 The value of J adopted in these calculations is that given by Griffiths and used in working out the experiments on benzene. Even if not correct, it is still the right value to use here, because it was determined by means of the same standards as those by which the quantity of heat developed in the benzene experiments was determined, so that any errors would eliminate. The other data were experimental. The agreement between the found and calculated values of L is fairly close in most of the cases examined. The work is still in progress. 5. On a Harmonic Analyser. By G. U. YULE. The author exhibited the instrument, which is fully described in the 'Philosophical Magazine' for April 1895. TUESDAY, SEPTEMBER 17. The following Papers and Reports were read :- 1. On the Electrification and Diselectrification of Air and other Gases. By Lord KELVIN, MAGNUS MACLEAN, and ALEXANDER GALT. § 1. Experiments were made for the purpose of finding an approximation to the amount of electrification communicated to air by one or more electrified needle points. The apparatus consisted of a metallic can 48 cm. high and 21 cm. in diameter, supported by paraffine blocks, and connected to one pair of quadrants of a quadrant electrometer. It had a hole at the top to admit the electrifying wire, which was 5'31 metres long, hanging vertically within a metallic guard tube. This guard tube was always metallically connected to the other pair of quadrants of the electrometer and to its case, and to a metallic screen surrounding it. This prevented any external influences from sensibly affecting the electrometer, such as the working of the electric machine which stood on a shelf 5 metres above it. § 2. The experiment is conducted as follows:-One terminal of an electric machine is connected with the guard tube and the other with the electrifying wire which is let down so that the needle is in the centre of the can. The can is temporarily connected to the case of the electrometer. The electric machine is 1 Griffiths and Marshall. 2 Phil. Trans., 184 A (1893). then worked for some minutes, so as to electrify the air in the can. As soon as the machine is stopped the electrifying wire is lifted clear out of the can. The can and the quadrants in metallic connection with it are disconnected from the case of the electrometer, and the electrified air is very rapidly drawn away from the can by a blowpipe bellows arranged to exhaust. This releases the opposite kind of electricity from the inside of the can, and allows it to place itself in equilibrium on the outside of the can and on the insulated quadrants of the electrometer in metallic connection with it. § 3. We tried different lengths of time of electrification, and different numbers of needles and tinsel, but we found that one needle and four minutes of electrification gave nearly maximum effect. The greatest deflection observed was 936 scale divisions. To find, from this reading, the electric density of the air in the can, we took a metallic disc, of 2 cm. radius, attached to a long varnished glass rod, and placed it at a distance of 145 cm. from another and larger metallic disc. This small air condenser was charged from the electric light conductors in the laboratory to a difference of potential amounting to 100 volts. The insulated disc thus charged was removed and laid upon the roof of the large insulated can. This addition to the metal in connection with it does not sensibly influence its electrostatic capacity. The deflection observed was 122 scale divisions. The capacity of the condenser is approximately The quantity of electricity = π × 22 with which it was charged was X 1 145' 1:45 300 4:35 electrostatic unit. Hence the quantity to give 936 scale divisions was X = 1.7637. = 1 936 4:35 122 The bellows was worked vigorously for two and a half minutes, and in that time all the electrified air would be exhausted. The capacity of the can was 16,632 cubic centimetres, which gives, for the quantity of electricity per cubic 1.7637 centimetre, = 1·06 × 10-4. The electrification of the air in this case was 16632 positive it was about as great as the greatest we got, whether positive or negative, in common air when we electrified it by discharge from needle points. This is about four times the electric density which we roughly estimated as about the greatest given to the air in the inside of a large metal vat, electrified by a needle point and then left to itself, and tested by the potential of a water-dropper with its nozzle in the centre of the vat, in experiments made two years ago, and described in a communication to the Royal Society of date May 1894.1 § 4. In subsequent experiments electrifying common air in a large gas-holder over water by an insulated gas flame burning within it with a wire in the interior of the flame kept electrified by an electric machine to about 6,000 volts, whether positively or negatively, we found as much as 15 × 10- for the electric density of the air. Electrifying carbonic acid in the same gas-holder, whether positively or negatively, by needle points, we obtained an electric density of 2.2 × 10−1. $5. We found about the same electric density (22 × 10-4) of negative electricity in carbonic acid gas drawn from an iron cylinder lying horizontally, and allowed to pass by a U-tube into the gas-holder without bubbling through the water. This electrification was due probably not to carbonic acid gas rushing through the stopcock of the cylinder, but to bubbling from the liquid carbonic acid in its interior, or to the formation of carbonic acid snow in the passages and its subsequent evaporation. When carbonic acid gas was drawn slowly from the liquid carbonic acid in the iron cylinder placed upright, and allowed to pass, without bubbling, through the U-tube into the gas-holder over water, no electrification was found in the gas unless electricity was communicated to it from needle points. § 6. The electrifications of air and carbonic acid described in Sections 4 and 5 were tested, and their electric densities measured, by drawing by an air pump 1 On the Electrification of Air. By Lord Kelvin and Magnus Maclean. 1 a measured quantity of the gas from the gas-holder through an indiarubber tube to a receiver of known efficiency and of known capacity in connection with the electrometer. We have not yet measured how much electricity was lost in the passage through the indiarubber tube. It was not probably nothing; and the electric density of the gas before leaving the gas-holder was no doubt greater, though perhaps not much greater, than what it had when it reached the electric receiver. § 7. The efficiency of the electric receivers used was approximately determined by putting two of them in series, with a paraffine tunnel between them, and measuring by means of two quadrant electrometers the quantity of electricity which each took from a measured quantity of air drawn through them. By performing this experiment several times, with the order of the two receivers alternately reversed, we had data for calculating the proportion of the electricity taken by each receiver from the air entering it, on the assumption that the proportion taken by each receiver was the same in each case. This assumption was approximately justified by the results. § 8. Thus we found for the efficiencies of two different receivers respectively 0.77 and 0.31 with air electrified positively or negatively by needle points; and 0.82 and 0 42 with carbonic acid gas electrified negatively by being drawn from an iron cylinder placed on its side. Each of these receivers consisted of block tin pipe 4 cm. long and 1 cm. diameter, with five plugs of cotton wool kept in position by six discs of fine wire gauze. The great difference in their efficiency was, no doubt, due to the quantities of cotton wool being different, or differently compressed in the two. § 9. We have commenced, and we hope to continue, an investigation of the efficiency of electric receivers of various kinds, such as block tin, brass, and platinum tubes from 2 to 4 cm. long and from 1 mm. to 1 cm. internal diameter, all of smooth bore and without any cotton wool or wire gauze filters in them; also a polished metal solid insulated within a paraffine tunnel. This investigation, made with various quantities of air drawn through per second have already given us some interesting and surprising results, which we hope to describe after we have learned more by further experimenting. § 10. In addition to our experiments on electric filters we have made many other experiments to find other means for the diselectrification of air. It might be supposed that drawing air in bubbles through water should be very effective for this purpose, but we find that this is far from being the case. We had previously found that non-electrified air drawn in bubbles through pure water becomes negatively electrified, and through salt water positively. We now find that positively electrified air drawn through pure water, and negatively electrified air through salt water, has its electrification diminished but not annulled if the primitive electrification is sufficiently strong. Negatively electrified air drawn in bubbles through pure water, and positively electrified air drawn through salt water, has its electrification augmented. § 11. To test the effects of heat we drew air through combustion tubes of German glass about 180 cm. long and 24 or 14 cm. bore, the heat being applied externally to about 120 cm. of the length. We found that when the temperature was raised to nearly a dull red heat, air, whether positively or negatively electrified, lost little or nothing of its electrification by being drawn through the tube. When the temperature was raised to a dull red heat, and to a bright red, high enough to soften the glass, losses up to as much as four-fifths of the whole electrification were sometimes observed, but never complete diselectrification. The results, however, were very irregular. Non-electrified air never became sensibly electrified by being drawn through the hot glass tubes in our experiments; but it gained strong posi The gas-holder was 38 cm. high and 81 cm. in circumference. Ten strokes of the pump raised the water inside to a height of 8.1 cm., so that the volume of air drawn through the receivers in the experiments was 428 cubic cm. per stroke of the pump. This agrees with the measured effective volume of the two cylinders of the pump. tive electrification when pieces of copper foil, and negative electrification when pieces of carbon, were placed in the tube, and when the temperature was sufficient to powerfully oxidise the copper or to burn away the charcoal, $12. Through the kindness of Mr. E. Matthey, we have been able to experiment with a platinum tube 1 metre long and 1 mm. bore. It was heated either by a gas flame or an electric current. When the tube was cold, and non-electrified air drawn through it, we found no signs of electrification by our receiver and electrometer. But when the tube was made red or white hot, either by gas burners applied externally or by an electric current through the metal of the tube, the previously non-electrified air drawn through it was found to be electrified strongly positive. To get complete command of the temperature we passed a measured electric current through 20 cm. of the platinum tube. On increasing the current till the tube began to be at a scarcely visible dull red heat we found but little electrification of the air. When the tube was a little warmer, so as to be quite visibly red hot, large electrification became manifest. Thus 60 strokes of the air-pump gave 45 scale divisions on the electrometer when the tube was dull red, and 395 scale divisions (7 volts) when it was a bright red (produced by a current of 36 ampères). With stronger currents, raising the tube to white-hot temperature, the electrification seemed to be considerably less. 2. Do Vertical (Earth-Air) Electric Currents Exist in the United In a paper by Dr. Adolph Schmidt read before Section A of the British Association at Oxford,' the author stated that he had expanded the components of the earth's magnetic force in series, and had deduced expressions, two of which give the magnetic potential on the surface of the earth in so far as it depends on (1) internal and (2) external forces. The third series represents that part of the magnetic forces which cannot be expressed in terms of a potential, but must be due to electric currents traversing the earth's surface.' The author concludes that such currents amount on the average to about 01 ampere per square kilometre.' It appeared, therefore, desirable that this conclusion, drawn from the magnetic state of the earth as a whole, should be tested by means of those portions which have been most fully studied. The test to be applied is whether the line integral of the magnetic force taken round a re-entrant circuit is or is not a vanishing quantity. The irregular form of the United Kingdom makes the application of this test more difficult than it would otherwise be, but as two detailed surveys of Great Britain and Ireland have been carried out by Dr. Thorpe and myself for the epochs 1886 and 1891 respectively, the data at our disposal are so numerous that I thought it worth while to undertake the inquiry. The actual work of calculation has been carried out almost entirely by two of my students, Messrs. Kay and Whalley. My best thanks are due to them for the care and skill they have displayed. Two circuits called the a and ẞ circuits respectively were selected, bounded by the following lines : (a) Long. 2° W., Lat. 58° N.; Long. 7° W., Lat. 52° N. The work done by a unit magnetic pole on traversing these circuits was calculated for the epoch 1886.0 by means of the terrestrial lines found for that date, and also for the epoch 1891.0 by means (1) of the same lines when due allowance was made for the secular change, and (2) of the independent set of lines found by aid of the later survey. The magnitudes of the hypothetical currents deduced from these calculations Rep. Brit. Assoc., 1894, p. 570. |