600 SCIENTIFIC RECOno EOR 188r,. 



eter, kumiuatrng in t be bulb, which lias a somewhat heavier wire sealed 

 into it. The resistance between the upper and lower ends of the ther- 

 mometer will depend largely on the length of the exposed j)latinum wire, 

 and this upon the temperature ; so that as the temperature rises the re- 

 sistance in circuit is diminished by an amount equal to the difference be- 

 tween that of the platinum wire which disappears and that of the mercury 

 which takes its place less the increase in the resistance of the wire and 

 mercury due to increase of temperature. From the equation represent- 

 ing this change in resistance as a function of the temperature a curve 

 is constructed from which the temperature corresponding to auy given 

 resistance may easily be read. The resistance is measured on a Wheat- 

 stone's bridge, the telephone being conveniently substituted for the 

 galvanometer for ordinary work. {Am. J. Sci., August, 1885, III, xxx, 

 114.) 



Angstrom's geothermometer consists of an ordinary instrument whose 

 bulb is placed in an iron vessel containing mercury sunk to the required 

 depth. The stem is open above and in it hangs a metallic wire moved 

 at the surface of the ground by means of a rack and pinion. So soon 

 as the wire touches the mercury an electric circuit is closed, and this, 

 by means of an electro-magnet, arrests the pinion. The stem carries a 

 graduated scale previously calibrated, so that a simple inspection gives 

 the temperature at once. No correction for temperature is necessary, 

 since the parts are compensated. (J. Phys., January, 1885, II, iv, 46.) 



Whipple has communicated to the London Physical Society a de- 

 scription of the process followed at Kew for testing thermometers at or 

 near the melting point of mercury. About 20 pounds of mercury are 

 poured into a wooden bowl and frozen by carbon dioxide- snow and 

 ether. The mercury is stirred with a wooden stirrer and the snow is 

 added till the experimenter feels, by the resistance to stirring, that the 

 mercury is freezing. By continuing the stirring for some time the 

 mercury becoms a granular instead of a solid mass. The thermometers 

 are then inserted together with a standard and compared. About one 

 hundred mercury or forty spirit thermometers can be thus examined in 

 half an hour, using about 200 gallons of carbon dioxide gas compressed 

 to form the snow. The bowl, ether, and mercury are cooled first to 

 — -10° by an ordinary freezing mixture. The average correction at the 

 melting point of mercury is now less than 1° F. When the process 

 was introduced, in 1872, it amounted to 5°, but has steadily decreased. 

 {Nature, November, 1885, xxxiii, 93.) 



2. Expansion and Change of State. 



Madan, doubting the statement generally made that stretched india- 

 rubber forms an exception to the general law of expansion by heat, has 

 called attention to Russner's investigations on this subject made in 1882. 

 He finds (1) that india-rubber always has a definite and positive coeflfi 



