absorbed. A streak of azure-blue water outside the green grotto showed the absorption bands previously observed. The red grotto showed no trace of red light. The spectroscope showed merely the absorption appearances of ordinary sea water. An examination of the Swiss ice holes by the spectroscope showed an absorption of the red end of the spectrum which gradually diminished toward the yellow. No especial or marked absorption bands were noticed. The Yellowstone Springs showed in a pronounced manner Schönn's aqueous bands in the red and red yellow. Ann. der Physik und Chemie, No. I, 1895, p. 175-177. J. T. 7. The minimum temperature of visibility.-A recent paper by P. L. GRAY describes experiments made upon a strip of platinum with the object of determining the minimum temperature at which it becomes visible in the dark. The author refers to the paper by Draper* as giving the only exact results upon the subject. He shows that Draper's temperature of minimum visibility, corrected by recent determinations of the coefficient of expansion of platinum, becomes 490° C., instead of 525°, and is not very much above his own determination given below. Furthermore Draper's conclusion that all solid bodies become visible at the same temperature is fully confirmed by the author's observations with bright and lamp-blacked platinum. In order to determine the temperature of the platinum strip Gray used a modified form of Joly's meldometer,t consisting essentially of a strip of very thin platinum, about 10cm long, 1c broad and mm thick, placed in a vertical plane. In regard to its use the author says: "It can be heated by an electric current, and its linear expansion is indicated by an optical method, by which an alteration in temperature of 1° can easily be noticed. The method of calibration is described in Joly's paper, and in that already mentioned, so that it is unnecessary to do more than briefly refer to it here. Minute fragments of substances of known melting-points are placed on the strip and watched through a microscope, while the temperature is very slowly and cautiously raised until, in any case, melting is seen to take place, when the position of the spot of light which indicates the expansion of the strip is noted. In these experiments the substances used were K,NO,(339°), AgCl (451°), KBr (699°), and gold (1041°). From these observations a curve showing the relation between temperature and scale-readings is obtained." "Method of Making the Experiments.-The first requisite was to get the strip in a perfectly dark enclosure, within which both eyes could be directed towards it without strain. To this end the apparatus was enclosed in a wooden box (blackened within), one end of which was replaced by a black velvet cloth, under which the observer placed his head, and which he could gather round *On the production of light by heat, Phil. Mag., xxx, 345, 1847. his neck and under his chin so that not a ray of light could penetrate the enclosure. The box was about 48cm long, 30 broad, and 22 high, and ordinarily the eyes, in making an observation, would be about 30cm from the strip. The other end of the box was provided with a hinged shutter, which was lifted immediately after an observation had been made, for the purpose of noting the temperature of the strip. The strip itself was further protected from draughts, etc., by means of a piece of brass, bent twice at right angles, and resting on the slate block below the strip, as in the calibration experiments. The angular dimensions of the surface of platinum, as seen in any experiment, were therefore: Apparent length = 3° 49' approximately; 66 width = 1 54 so that the apparent area subtended was about 36 times that of the full moon. The current by which the strip was heated ran through a variable carbon resistance, the handle of which was within convenient reach of the observer as he sat with his head under the black cloth. He could thus alter the temperature of the platinum until it was on the very verge of invisibility, a very small fraction of a turn being then sufficient to produce utter darkness where before the area of faint light had been. A contact-breaker was also within convenient reach, so that the current could be broken or made at pleasure, and the objective reality of the faint luminosity at the limiting-point thus demonstrated. When he was satisfied that the limiting point had been reached the hinged end of the box was opened, a beam of light sent to the mirror connected with the strip, and the deflection, giving the temperature, read on the scale. The possible error in the estimation of the absolute value of the temperature may be taken as certainly not more than 2°.... The general conclusions reached are as follows: "(1) That the minimum temperature of visibility is the same. for a bright polished metallic surface as for one covered with lampblack, although the intensity of the radiation in the two cases may be different. This result may at first be, to some, unexpected, but a little consideration will show that it might have been, à priori, anticipated. For probably temperature governs the highest wavelength from a radiating body, and wave-length governs visibility, at least after an extremely small intensity of radiation has been passed. (2) That the visible limit at the red end of the spectrum varies greatly for a normal eye, according to its state of preparation, i. e. according to the intensity of the light in which the observer has been before making the observation. . . . Speaking generally, we may say that a bright light diminishes. the sensitiveness of the eye to radiation of low frequency; that AM. Jour. Sci.—Third Series, VOL. XLIX, No. 291.—MARCH, 1894. darkness increases it. Or that, as a rule, the eye is less sensitive in the morning than at night. (3) That for the less sensitive condition, the minimum temperature of visibility for the surface of a solid is about 470° C., but that this may be much reduced by even a few minutes in a dark room. (4) That at night, a surface at a temperature of 410° is visible, and that by resting the eyes in complete darkness, this may be reduced to as low as 370° nearly, below which apparently one cannot go, since 10 minutes' rest appears to be almost as efficacious as 3 hours'. (5) That different people's eyes (of no special or known departure from normality) differ somewhat in their 'minimum tem· perature of visibility,' but probably not to any great extent, if tested under the same conditions as to preparation, etc. . . The loss of distinct color at the low temperatures is very striking; the appearance to the author, and to most of the observers, has absolutely nothing of red in it, but is like a white mist-the nearest comparison that can be made. In the morning observations, however, when the strip disappeared at from 460° to 470°, the last appearance was distinctly reddish; and this agrees with one observation noted at night, when after getting the visibility critical-point at about 390° C., the temperature was raised until one could declare for certain that the light looked red: it was then found to be 449°. Of course, in all the observations, the luminous area was most distinctly seen by somewhat averting the gaze from it; generally it was found best to look in the direction of either far upper corner of the enclosure. As already mentioned, most of the observers pronounced the appearance at the critical-point to be that of a whitish mist;" one, however, thought he saw a slight lilac tinge' in it; and 'Case G' declared it to be decidedly yellow, which is interesting, because to him a red mark on white paper (such as a pip on a card belonging to one of the red suits of a pack) appears yellow, by artificial light at night. In one experiment a plate of glass, inch thick, and in another a layer of water, inch thick, were inserted between the strip and the eye, without making the slightest difference in the phenomena; showing (1) that the point where these substances begin to be more or less opaque to infra-red radiation had not been reached; (2) that the small difference in intensity produced by their insertion had no appreciable effect. This last conclusion is far more strongly borne out by the equality of temperature in the case of the bare metallic and the black surfaces, and indicates that in all the cases it was wave-length, and not intensity, which was determinative of visibility, so disposing of the possible objection that the difference between 'morning' and 'evening' might be due merely to the state of enlargement of the pupil of the eye, which would naturally be more contracted at the one time than at the other, thus affecting the total amount of radiation falling on the retina. Also, if such an objection were valid, it would imply that fatigue of the muscles of the iris produced a relatively enormous time-lag' in following changes of luminous intensity, which we know does not exist. There seems, in fact, to be little doubt that the difference is due to the retina itself becoming sensitive to long waves after rest, which were incapable of affecting it when it was in some way fatigued by exposure to the ordinary bright light of day. The next and obvious step is to find the respective wave-lengths corresponding to the different temperatures. This point, however, and others, cannot be determined without some additions to the present apparatus, and will form the subject of a future paper." -Proc. Phys. Soc., London, xiii, 122. 8. On the liquefaction of air. A note communicated by Professor GEORGE DAVIDSON. The recent remarkable experiments of Professor Dewar in liquefying air, etc., recall the experiments of Perkins in 1822-1826 as detailed in a paper of the Royal Society read June 15, 1826 (p. 541). Mr. Perkins describes the apparatus which he had devised and operated; and says, "this tube [of steel] I filled with water and. subjected it to a pressure of 2,000 atmospheres. After repeating this experiment a great number of times, the average of the result showed that the column of water, 8 inches long, was compressed of an inch, or part of its length." . . . "With the same apparatus I also made experiments on the compression of other fluids. The most remarkable result I obtained was with concentrated acetic acid; which, after compression with a force of 1100 atmospheres, was found to be beautifully crystallized, with the exception of about part of fluid, which, when poured out, was only slightly acid. As it might be supposed that even glass was pervious to water by such a force, [500 atmospheres,] a small phial was made airtight, by fitting into its neck a well-ground glass stopper. It sustained pressure of 500 atmospheres without change and was perfectly dry within, although it remained under that pressure 15 minutes. It was next subjected to a pressure of 800 atmospheres, and when taken out was found to be crushed to atoms. "In the course of my experiments on the compression of atmospheric air, by the same apparatus that had been used for compressing water, I observed a curious fact, which induced me to extend the experiment; viz., that of the air beginning to disappear at a pressure of 500 atmospheres, evidently by partial liquefaction, which is indicated by the quicksilver not settling down to a level with its surface. At an increased pressure of 600 atmospheres, the quicksilver was suspended about of the volume up the tube or gasometer; at 800 atmospheres, it remained about up the tube; at 1000 atmospheres, up the tube, and small globules of liquid began to form about the top of it; at 1200 atmospheres, the quicksilver remained up the tube, and a beautiful transparent liquid was seen on the surface of the quicksilver, in quantity about zoo part of the column of air. The gasometer was at another time charged with carburetted hydrogen, and placed in the receiving tube with its mouth immersed in the quicksilver; it was subjected to different pressures, and it began to liquefy at about 40 atmospheres, and at 1200 atmospheres the whole was liquefied. "These instances of apparent condensation of gaseous fluids were first observed in January, 1822; but for want of chymical knowledge requisite to ascertain the exact nature of the liquids produced, I did not pursue the inquiry further," etc., etc. 9. On the Value of μ for rapid Electrical Oscillations; by CHARLES E. ST. JOHN (communicated).-In my paper "On WaveLengths of Electricity on Iron Wires," in a recent number of this Journal (vol. xlviii, 311). I gave as a by product of my investigation the value μ 385. Various estimates have been made for this quantity, but there has been a lack of experimental data. A late paper by Ignaz Klemencic in Wiedemann's Annalen, No. 12, 1894, contains the following values of the permeability in case of oscillations of 100,000 per second: μ This value of μ led me to reëxamine my own results, and recalculation shows an arithmetical error, by which the values were multiplied by 4, so that the true approximate value yield by the data is 96, the separate values being μ = 107, μ = 97·5, μ = 83.5 for the different specimens of iron. These are somewhat lower than the results found by Klemencic, but in my experiments the rate of oscillation was much higher. In this connection a remark made by Mr. Oliver Heavyside in his Electrical Papers, vol. i, p. 361, is interesting. He says that u is eminently variable but that μ = 100 is a fair average value. Both Klemencic's results and my own confirm this assumption. Berlin, Jan. 10, 1895. 10. National Academy of Sciences on Electrical Measurement.-In July, 1894, an act of Congress was passed to define and establish the units of electrical measure. By this law, Congress made it the duty of the National Academy of Sciences to prescribe and publish specifications necessary for the practical application of the definitions of certain units of electrical measure adopted in the act. This law (H. R. 6500), approved July 12, 1894, is as follows: AN ACT TO DEFINE AND ESTABLISH THE UNITS OF ELECTRICAL MEASURE. Be it enacted by the Senate and House of Representatives of the United States of America in Congress assembled, That from and after the passage of this Act the legal units of electrical measure in the United States shall be as follows: First. The unit of resistance shall be what is known as the international ohm, which is substantially equal to one thousand million units of resistance of the centimeter-gram-second system of electro-magnetic units, and is represented by the resistance offered to an unvarying electric current by a column of mercury at |