velocity much higher than that of sound. When 

 air is shut off by a collodion diaphragm at the 

 end of a tube, and is compressed by a piston, the 

 wave generated when the collodion bursts has a 

 ery high velocity. Some such diaphragms will 

 turst at a pressure of 27 atmospheres, and the 

 ave is then propagated faster than 600 metres 

 r second. The rate decreases by about 20 

 metres per second for every metre traversed, until 

 it becomes the ordinary velocity of sound. (See 

 so Temperature of Flames, under HEAT). 

 Efflux of Gases. Emden (Wiedemann's An- 

 len, October, 1899) has developed the mathe- 

 atical theory of the efflux of gas under pressure, 

 tarting from the position that, if the efflux is 

 ,eady, equal masses and equal quantities of en- 

 gy must pass the space between two cross sec- 

 ions of the tube in each unit of time. He arrives 

 at the following important generalizations: The 

 velocity with which the gas passes the orifice 

 ,n nevr become greater than the velocity of 

 und at that point. The pressure at which a 

 t begins to pass the orifice with this velocity 

 rresponds to the velocity of efflux at which sta- 

 onary sound waves are formed in the jet. In 

 is manner sound waves of very small wave 

 :ngth and very high frequency may be formed, 

 possessing greater energy than any yet produced. 

 Acoustics. Velocity of Sound. Witkowski 



R Bulletin of the Cracow Academy of Science, 16, 

 . 138), by experimenting with compressed air 

 y Kundt's method at various temperatures and 

 pressures, has shown that the velocity of sound 

 aries with the pressure, the variation being 

 eatest at the lowest temperatures. At zero the 

 elocity slowly rises with the pressure, being 

 ,bout 10 per cent, higher at 100 atmospheres than 

 .t 1 atmosphere. At 100 the velocity remains 

 early the same at all pressures, while at 130 

 rapidly falls with increasing pressure. Bril- 

 uin (Annales de Chimie et de Physique, 18, 1899) 

 ows mathematically that when the transmis- 

 ion of sound with its condensations and rarefac- 

 tions takes place in a mixture of gases, gaseous 

 diffusion must be taken into account, as it causes 

 variations in composition of the medium. 



Pitch. Koenig (Wiedemann's Annalen, No- 

 vember and December, 1899) finds that Kundt's 

 dust figures can be used to determine pitches 

 high above the limits of audibility. The fork c 7 , 

 placed at the mouth of a tube with a length of 

 some 95 semi-wave-lengths, and a diameter of 

 about one semi-wave-length (11 millimetres) 

 gives very clearly defined figures. The method is 

 available up to f" (90,000 complete vibrations per 

 second), a pitch which is more than an octave 

 bove the extreme limit of audibility. The meth- 

 of beats ceases to be practicably available two 

 taves below that limit. 



Plionofjrapliy. Mauro (Journal of the Frank- 

 Institute, July) describes a modification of 

 11 and Tainter's graphophone in which undula- 

 ions of greater amplitude are insured by increas- 

 ing the velocity of the style. The style is set 

 at a more acute angle, and the diameter of the 

 cylinder is increased. The increase in velocity 

 also prevents the shank of the style from coming 

 into contact with the wax and so checking the 

 vibration. 



Heat. Tliermometry. Holborn and Day 

 (American Journal of Science, September, 1899) 

 find that up to 500 C. thermometer bulbs of 

 Jena borosilicate glass No. 59 111 with inclosed hy- 

 drogen prove most satisfactory, no appreciable 

 changes in the zero point being shown after re- 

 peated heatings. Callendar (Philosophical Maga- 

 zine, December, 1899) advocates the adoption of 



PHYSICS, PROGRESS OF, IN 1900. 



569 



a new practical temperature scale, to be known 

 as the " British Association Standard." Accord- 

 ing to this standard any temperature, t, would be 

 defined by the equation t pt=dt(t 100), 

 where pt = WO (R K )/(R 100 R ) and d is a 

 constant, R, R , and R 100 being the electrical re- 

 sistances of a particular platinum wire at t, 

 and 100 C. respectively, and the value of d being 

 obtained by taking the boiling point of sulphur 

 under 76 centimetres of mercury at C. as 

 444.53 C. This scale is subject to far less un- 

 certainty, especially at high temperatures, than 

 that practically obtained with gas thermometers 

 as at present constructed and used; it is easily 

 and accurately reproducible, and is a close ap- 

 proximation to Kelvin's absolute scale. 



Conduction. Smolan (Vienna Academy) states 

 that at the boundary between a solid and gas, 

 across which a flow of heat takes place, there is 

 an abrupt change, of temperature. When the 

 whole temperature difference is small this may 

 be expressed as an imaginary length added to the 

 thickness of gas through which conduction is 

 taking place. These lengths in different gases are 

 proportional to the mean free path, being 6.98 

 times as long for hydrogen as for carbonic acid, 

 and 1.70 time for air. 



Radiant Heat. Planck (Annalen der Physik, 

 April) endeavors to find an expression for the 

 entropy of radiant heat which is in agreement 

 with all the data of thermodynamics and of the 

 electromagnetic theory of light. He also arrives 

 at a numerical value of the temperature of a 

 monochromatic radiation emitted by a small sur- 

 face and refracted by a system of centric sur- 

 faces. The temperature of the radiation is com- 

 pletely defined without reference to that of the 

 body which emits it, or to the losses suffered on 

 the way. It is more rational, the author thinks, 

 to speak of the temperature of a monochromatic 

 beam of sunlight than of the temperature of the 

 sun. Liesegang (Physikalische Zeitschrift, April 

 14) has succeeded in making a paper sensitive to 

 long heat waves by painting it with equal quanti- 

 ties of hydroquinone and anhydrous sodium car- 

 bonate, mixed with a little alcohol. On exposure 

 to the radiation of a gas stove the blue color of 

 this paper is completely bleached in five seconds, 

 and shadow pictures of coins can be obtained. 

 Kurlbaum (Annalen der Physik, July) finds that 

 the temperature of the surface of a radiating 

 body is by no means that of the interior, as is 

 usually assumed. With a sheet of blackened 

 platinum foil Ijt thick the difference of tempera- 

 ture is 0.016 when the sheet is heated 4 above 

 its surroundings. 



Convection. Benard (Comptes Rendus, April 9 

 and 17) finds that a thin layer of liquid heated 

 uniformly from below may assume a stable condi- 

 tion in which it becomes divided into regular 

 polygonal prismatic cellules, in each of which the 

 liquid ascends along the axis and descends along 

 the outside. The forms have remarkable per- 

 manence and perfection. 



Specific Heat. Magie (Physical Review, Au- 

 gust, 1899) shows theoretically that the osmotic 

 pressure of a solution is equal to its latent heat 

 of expansion, and also points out the special 

 cases (four in number) where the specific heats 

 of the solvent and of the solute in a solution are 

 constant at all concentrations. 



Ebullition. Speyers (American Journal of Sci- 

 ence, May) shows that if we know the molecular 

 aggregations of the constituents in a homogene- 

 ous mixture of two liquids it is possible to plot- 

 the boiling-point curve of such a mixture. The 

 equation employed is n/(N + n) = (p p')/'d 





