and nearer to the negative one. The latter begins to redden; at first the blue light continually grows fainter, then becomes invisible; at least we cease to distinguish the shell that it forms around the wire; and if any trace of it remains, it is only a bluish tint in the light due to the incandescence. When the wires are almost in contact, especially if the finger is presssd lightly on the hammer of the contact-breaker, the incandescence of the negative wire becomes dazzling, and then there is no more appearance of the blue light. I was curious to know if it had really disappeared, or if it was only concealed by the brilliancy of the wire when white-hot; and I thought that the now celebrated method by which we discover the trace of the solar protuberances amongst the intenser rays of his disk might be applied here. I made use of a vertical spectroscope by Duboscq. The slit is vertical, and can be moved from one wire to the other along the spark. The characteristics of the spectrum change according as we view the brilliant point where the spark is detached from the positive wire, or the blue shell which envelopes the extremity of the negative wire, or, finally, if that is incandescent, the red parts which lie beyond the blue shell. We keep the slit upon the blue shell while the spark is too long to admit of the wire becoming red. The spectrum is characterized chiefly by a group of four green rays, a group of two rays placed between the green and the blue, a group of three violet rays, beyond which we can see others of less brilliancy. As before, we gradually bring the positive wire nearer to the negative wire, which latter begins to redden. One would expect to see a continuous spectrum; and this in fact is what actually happens, if we direct the slit towards the parts of the red wire which are beyond the blue electric glow. We have then a continuous spectrum which is worth noting, because we thus learn, without requiring to light up the micrometric scale, that the violet rays given by the blue light correspond nearly to the most refrangible extremity of this continuous spectrum. Bringing back the slit to the extreme end of the negative thread, we find again the streaked spectrum of the blue light. The red in it becomes more brilliant in proportion as the thread becomes more incandescent; but the green, blue, and violet rays still continue. But when the incandescence is very intense, the green rays disappear, then the blue, and the spectrum is continuous into the violet, but at the extremity of the violet we still perceive the group of three violet rays, which become less distinct, but mark their position until the thread begins to melt. The ultra-violet rays have ceased to be visible. Thus the spectroscope permits us in this case, as well as in the observation of the solar protuberances, to ascertain the presence of a feeble glow in the midst of a light which to the direct vision is dazzling.-Comptes Rendus, June 7, 1869. ON THE MEAN VELOCITY OF THE MOTION OF TRANSLATION OF THE MOLECULES IN IMPERFECT GASES. BY M. P. BLASERNA. We are often led to inquire whence arise the deviations from Mariotte's law that experiment reveals in the different gases. I do not think that we can accept the explanation that M. Dubrunfaut has lately offered*, an explanation which tends to ascribe these deviations to small quantities of aqueous vapour existing in even the most perfectly dried gases. When Plücker published his experiments on Geissler's tubes, I succeeded in preparing tubes of nitrogen and of carbonic acid which contained no traces whatsoever of the three brilliant rays which belong to hydrogen and aqueous vapour. To accomplish this, I made use of a good common air-pump; I exhausted the receiver thirty or forty times, and I dried the gases by the ordinary means, except only that the electrodes were of platinum instead of aluminium, which is very often employed. This is the method, pointed out by Rudberg, that M. Regnault and all experimenters have followed. If, nevertheless, a trace of water does remain, it seems to me impossible that it should produce the great deviations that we observe in the case of imperfect gases. I have also proved that for air and carbonic acid the molecular state cannot be considered to result solely from mutual attractions or repulsions, whatever may be the law of these actions; in short, a cold and expanded gas, being then heated and compressed to the same volume, ought to exhibit the same phenomena with regard to its compressibility, which is contrary to experience. And the researches of M. Amagat have lately proved the same thing for ammonia and sulphurous acid. The mechanical theory of heat leads us, as a natural consequence, to regard heat as resulting from the motions of the molecules, and to define a gas as a body whose molecules travel in all directions in space. But MM. Krönig and Clausius have shown that if we suppose these progressive motions in the gas to be rectilinear, we arrive at Mariotte's law; and M. Clausius has even developed a formula which has enabled him to calculate the mean velocity of these motions for the better-known gases. The deviations from Mariotte's law arise consequently from attractions which still exist in the gases, and which are nothing but a particular case of universal attraction: these attractions are more or less feeble according to the mass and the mean distances, greater or less, by which the gaseous molecules are mutually separated. This is the simplest explanation we can offer of the phenomenon; it is the one which I believe is most generally accepted. All this being granted, we may determine the actual velocities of the molecules in imperfect gases. Imagine a kilogramme of gas, at temperature zero, and under an initial pressure p, so slight that the volume v shall be very great, so that we may disregard the attractions. Increasing the pressure to p, the volume will be v', and we shall have = 1+ Ap, Povo pv'。 = Ap being the deviation from Mariotte's law under the pressure p. Raising the temperature to t, the pressure being constant, the vo cient of expansion under a constant pressure between 0 and t, and for the pressure p. Putting Povoa 1+Ap a formula which combines the law of the compressibility and the law of the dilatation of imperfect gases, and in which R, and ap change with the pressure. Thus we have for R, the following values: p= 0 me- 0.76 1 Rp =29-222 29-325 29-347 29-672 30-007 30-265 30-446 = 19-329 19-388 19-437 20-417 21.907 23-867 25.915 Air u2 3g' But, according to M. Clausius, we have also pv= g being the acceleration due to gravity, and u the mean velocity of the progressive motion; then a formula which differs from that given by M. Clausius for perfect gases in that R, and ap are not constant, but functions of the pressure or volume. It may serve to determine the mean velocity of the molecules in the different gases. In the case of air and of carbonic acid, for which we have the requisite experimental data, we thus obtain the following velocities, expressed in metres per second : The velocities found for the pressure zero represent the ideal case of a gas infinitely rarefied (that is to say, perfect), the attractions being infinitely small. We see that the velocities diminish when the pressure increases-that is to say, when the volume becomes small and the attractions are more intense. For atmospheric air at 100° it is necessary to carry the calculation to the second decimal place in order to find the differences, which shows clearly the degree of perfection that this gas reaches at that temperature. It seems almost superfluous to remark that, in order that the numbers given for air may have a real significance, we must consider air, not as a mixture of two gases, but as a single ideal gas whose molecules possess the physical properties of nitrogen and of oxygen in known proportions. Comptes Rendus, July 12, 1869. XXXIX. Observations on the Temperature of the Human Body at various Altitudes, in connexion with the act of Ascending. By WILLIAM MARCET, M.D., F.R.S., Assistant Physician to the Hospital for Consumption and Diseases of the Chest, Brompton*. DURING an excursion over the Mont-Blanc range I had an opportunity this summer of ascertaining the temperature of my body under various circumstances connected with the act of ascending. The number of observations is, I must admit, much too small; still their individual results, when compared with each other, agree closely enough to allow of certain conclusions to be derived from them. I had with me a thermometer carefully made by Casella, and divided in fifths of degrees Centigrade, allowing of a tenth of a degree to be read off. The instrument could be accurately observed while its bulb was under my tongue, by means of a small mirror which, on being placed near the stem of the thermometer at a certain angle, reflected its image into my eyes, so that I could see the mercury rising or falling as plainly as if I was looking at it directly t. It is useless to add that in Communicated by the Author, having been read to the "Société de Physique et d'Histoire Naturelle of Geneva" on the 3rd of September, 1869. f On every occasion, I observed the height of the thermometer several times, and made sure of its being constant before noting the temperature, thus avoiding the fallacy which may easily arise from too short an exposure, as shown by Dr. Ch. Baeumler (Brit. Med. Journ. August 1869). Two observations made in London in the sitting posture, at 11 A.M. (breakfast at 9), with a thermometer constructed for me since my return by Casella, Phil. Mag. S. 4. Vol. 38. No. 256. Nov. 1869. Ꮓ these experiments the greatest care was taken to keep the bulb of the thermometer as far back as possible beneath the tongue, while the margin of that organ was applied firmly against the lower jaw, the lips being kept closed, so that respiration was entirely effected through the nose. It was, consequently, impossible that any of the air used for breathing could come into contact with the bulb of the thermometer while these observations were carried on. The questions which offered themselves for investigation related (1) to the influence of various degrees of altitude on animal heat, the body being in a state of rest; (2) to the influence of the act of ascending on animal heat observed at different heights; and (3) to the influence of the act of descending on the temperature of the body. I shall limit myself, on the present occasion, to the first two questions, leaving the influence of the act of descending on animal heat for future consideration. I soon ascertained the necessity of taking the temperatures while actually engaged in climbing; accordingly this was done. The instrument was withdrawn from its case and introduced under my tongue; the looking-glass was removed from the pocket, together with my watch; and the height of the thermometer was observed while in the act of ascending, and taking every care to slacken my speed as little as possible. I began by noticing that frequently, while ascending, the thermometer after a sufficiently long exposure showed a temperature which, though steady at the time, commenced rising shortly as nearly as possible on the model of that which I had used, gave the following results; the bulb was kept under the tongue, and the degrees read off with a looking-glass : Temperature. 2 36.5 |