Chemistry and Physics. 391 



vessel, protected from radiation, to a temperature below 0°, to 

 insert the thermometer, and then bring about the freezing of the 

 water by dropping in a crystal of ice. The thermometer then 

 rises, and finally attains a steady temperature, differing only very 

 slightly from the true zero. 



Within the space allotted to this abstract, it is not possible to 

 describe in detail all the precautions to be adopted and the appa- 

 ratus employed, and for these reference must be made to the 

 original paper. The following brief outline may, however, be 

 given. 



The apparatus consists of two portions, the thermostat and the 

 cooler. The former is a rectangular copper vessel, filled with 

 some liquid, which can be cooled below 0° without solidifying. 



Generally either refined petroleum or a strong solution of com- 

 mon salt is employed. This vessel communicates by means of two 

 wide tubes with a system of coils in the cooler, through which 

 the liquid can be pumped by a rotary stirrer. These coils are 

 surrounded by a freezing mixture at about —8°, and by this 

 means the circulating liquid can be cooled and maintained for 

 some time at about —2°. The distilled water to be frozen is con- 

 tained in a tube of about 800^° capacity made of clear glass. 

 This is first placed directly into the circulating liquid and cooled 

 quickly to — 0*5° or — 0'7°. It is then transferred to a copper 

 cylinder lined with polished metal, placed in the center of the 

 thermostat, an annular space of about 1^"^ being left between 

 them. The thermometer whose zero is to be taken is then quickly 

 fixed in position in a spring clamp, the bulb and a considerable 

 length of the stem above the zero being immersed in the water. 

 A crystal of ice is dropped in, and the temperature quickly rises 

 to the freezing point. 



For the details of the arrangement for the illumination of the 

 divisions, and taking the readings through the mass of the liquid 

 containing the ice crystals in suspension, reference must be made 

 to the paper. 



The amount of ice formed in the liquid varies of course with 

 the undercooling. Experiments made with good mercurial ther- 

 mometers showed that if ice be present in suflScient quantity, the 

 final temperature attained by the mixture of ice and water is not 

 influenced perceptibly by variation of the temperature of the 

 circulating liquid within fairly wide limits. As, however, it is 

 extremely doubtful whether the indications of any mercurial 

 thermometer can be relied on beyond 0'001°, it seemed desirable 

 to control this result by some other means. 



A platinum thermometer and bridge were therefore designed, 

 capable of indicating with certainty a change of 0'0001°, and a 

 description of the whole arrangement employed to attain this 

 degree of accuracy forms the second half of the paper. The 

 resistances in the bridge were of manganin, whose temperature 

 coefilcient is only about -^ that of the usual resistance alloys, and 

 the plugs usually employed for short circuiting the coils were 



