630 



PHYSICS, PROGRESS OP, IN 1898. 



Intensity. Reed (American Association) de- 

 scribes an instrument for measuring the intensity 

 of sound by observing the amplitude of vibration 

 of a diaphragm with a micrometer microscope fo- 

 cused on the tip of a stylus attached to the dia- 

 phragm. Webster (ibid.) measures the amplitude 

 of a thin glass diaphragm by the interferometer, 

 the fringes being photographe'd on a moving plate. 



Interference. Cremieu(Societe Francaisede Phy- 

 sique) has produced elliptical sound vibrations in 

 air by the interference of the longitudinal vibra- 

 tions of two organ pipes at right angles. By pro- 

 viding for the proper difference of phases an ellip- 

 tic motion, whose presence was proved by delicate 

 quartz fibers, was set up at their intersection. 



Heat. Thermometry. Barker (London Royal 

 Society) has devised a new method for the exact 

 determination of the zero point in thermometers. 

 Distilled water is cooled below zero (centigrade) in 

 a vessel protected from radiation ; then the ther- 

 mometer is introduced and freezing is induced by 

 dropping in an ice crystal. The thermometer rises 

 and finally attains almost exactly the true zero. 

 Measurements made with an electrical-resistance 

 thermometer show that the temperature of the mix- 

 ture into which the bulb is plunged varies not more 

 than TnBffTT to Trnhnr f a degree for a variation of 2 

 to 3 in the liquid that circulates in the apparatus. 

 Watson (" Philosophical Magazine '') describes an 

 apparatus to facilitate the comparison of thermom- 

 eters of very different dimensions. The two instru- 

 ments are inclosed in a glass tube and their reser- 

 voirs are plunged into mercury. The tube is 

 surrounded by a larger tube, and in the included 

 space a liquid such as carbon disulphide, ethyl al- 

 cohol, or chlorobenzene is boiled under any desired 

 pressure. Water can not be used because it con- 

 denses on the glass and makes reading difficult. 

 The liquid is boiled by electrically heating a plati- 

 num coil. With this device a temperature constant 

 to ritf of 1 can be maintained for three or four 

 hours. Half an hour is necessary for the establish- 

 ment of thermal equilibrium corresponding to a 

 given pressure. Richards (" American Journal of 

 Science," September) proposes to make use in ther- 

 mometry of the transition temperature or "melt- 

 ing point" of sodium sulphate (Na a S0 4 -f- 10H a O), 

 which is almost exactly 32.48". This point is easily 

 obtained and very constant. Uehling and Stein- 

 bart (" Revue Universelle des Mines," July) have 

 devised a pneumatic pyrometer that has been tested 

 and calibrated to 1,650. It consists of two cham- 

 bers through which an air current passes with con- 

 stant and uniform velocity. The air enters the first 

 chamber by an orifice, A, and the second through an 

 orifice, B. If the specific gravity of the air remains 

 the same, the same quantity will pass A and B. and 

 the pressure in the two chambers will be the same. 

 But if in any way, as by heating, the density of the 

 air in the first chamber is altered, the volumes 

 passing A and B will be different and the pressures 

 in the chambers will no longer be the same. By 

 registering this difference of pressure the tempera- 

 ture of the first chamber can therefore be ascer- 

 tained. Berthelot (Paris Academy of Sciences, Jan. 

 31) measures high temperatures by an interference 

 method based on the fact that if the density of a 

 gas is diminished to the same extent by rise of tem- 

 perature or by diminution of pressure, the index of 

 refraction has the same value m both cases. 



Conductivity. Straneo (" Atti dei Lincei," VII, 

 11) by using a method of determining simulta- 

 neously electric and thermal conductivities, finds 

 that the internal conductivity varies with the tem- 

 perature, but in a degree too small to be exactly de- 

 termined. The surface conductivity and the'dis- 

 persivity increase with the temperature. Hall 



(American Association) points out that the method 

 of measuring thermal conductivity in which thin 

 plates are used is unsatisfactory owing to the diffi- 

 culty of determining the difference of temperature 

 of the two faces. He obviates this by copper-plating 

 the two faces of his iron plate and by using the 

 copper-iron thermo-couples thus formed for deter- 

 mining the temperature difference. He finds that 

 the conductivity of iron increases as the tempera- 

 ture is decreased at the rate of about 1 per cent, for 

 8 C. Lees (London Royal Society, Dec. 16, 1897) 

 believes that he has established the following ex- 

 perimental laws connecting conductivity with tem- 

 perature : 



" 1. Solids, not very good conductors of heat in 

 general, decrease in conductivity with increase of 

 temperature in the neighborhood of 40 U. Glass is 

 an exception. 



" 2. Liquids follow the same law near 30 C. 



"3. The conductivity of a substance does not 

 always change abruptly at the melting point. 



" 4. The thermal conductivity of a mixture lies 

 between the conductivities of its constituents, but 

 is not a linear function of its composition. 



' 5. Mixtures of liquids decrease in conductivity 

 with increase of temperature near 30 C. at about 

 the same rate as their constituents. 



B. O. Peirce and Willson (American Academy of 

 Arts and Sciences, August), from a determination 

 of the thermal conductivities of about 20 varietie > 

 of marble, when the faces of the slabs are kept re- 

 spectively at 18 C. and 45 C. find that the con- 

 ductivity may depend to several per cent, on the 

 amount of absorbed moisture. The conductivities 

 ranged from 0.00501 to 0.00761. 



Convection Guebhard (" Nature," June 30) finds 

 that when a slightly fogged photographic plate is 

 developed in a shallow bath and the experimenter 

 presses his finger on the plate during the process 

 streaks are observed to radiate from the parts 

 touched. These are due to convection currents; 

 caused by the warmth of the finger, and can also be 

 produced by warmed inorganic bodies. These re- 

 sults are of interest because the streaks have been 

 represented by some to be photographs of the " lines 

 of force of animal magnetism." 



Specific Heat. Lummer and Pringsheim (Wiede- 

 mann's " Annalen ") have measured the ratio of the 

 two specific heats (at constant volume and constant 

 pressure) in certain gases, using a new form of bo- 

 lometer and employing the relationship between 

 temperature and pressure in adiabatic expansion. 

 The results are as follow : 



Air 1.4025 Carbonic acid 1.299) 



Oxygen 1.3977 Hydrogen l.iosi 



Leduc (Paris Academy of Sciences. Oct. 31) con- 

 cludes theoretically that the ratio of the two spe- 

 cific heats of air varies slightly with the tempera- 

 ture, the ratio between the value for and that for 

 100 being 1.0006. For carbon dioxide the same 

 ratio is 1.028. Behn (ibid., October) concludes thu: 

 if the decrease of the specific heat of metals with 

 the temperature be represented by curves the.-- 

 will all intersect at the absolute zero. It is possible 

 that all the specific heats themselves become zero 

 at this point, and in any case the law of Dulong ami 

 Petit does not hold for low temperatures. Tilden. 

 in a lecture before the Royal Institution. May 1'i 

 (''Science," July 29), describes experiments under- 

 taken to discover whether the law of Dulong and 

 Petit (that the product of atomic weight and specifn' 

 heat is constant) is or is not exact. Earlier experi- 

 menters had concluded that it is not, and Prof. 

 Tilden by using very pure metals sought to limi 

 whether their results were or were not due to im- 

 pure materials. His result for cobalt and nickel. 

 which make the specific heats respectively 0.1035 and 



