388 Dr. W. W. J. Nicol on the 



From the data thus obtained the bulbs were proportioned 

 to each tube, so that the value of each millim. in terms of the 

 total capacity of the bulb should lie between 0*00004 and 

 0*00006. When the bulbs had been sealed on, the dilato- 

 meters were filled with mercury at 20° C. up to the zero 

 mark on the stem, and the weight of the mercury was deter- 

 mined. In the case above this was 79*93 grm. Thus the 

 mean value of 1 millim. of the stem was 5*74, that of the bulb 

 and stem up to the zero being 100,000. This was then cor- 

 rected accordingto the calibration results for every 100 millim., 

 giving : — 



0-100 = 5-778 300-400 = 5*713 



100-200 = 5-713 400-500 = 5-701 



200-300 = 5-718 500-600 = 5-778. 



The coefficient of expansion of the glass was determined in 

 each case by both mercury and water. With the above dila- 

 tometer the apparent expansion of mercury between 20° C. 

 and 78°-8 C. was found to be 100,914-6. Calculated from 

 Landolt's tables the true volume is 101,069, difference 154*4. 

 The volume of water was 102,483*8, calculated 102,637*8, 

 difference 154*0, giving coefficient for glass = 0*00002*62. 



Of the various dilatometers thus made only three were used 

 in the following experiments. In these the mean value of 

 1 millim. of the stem was Di=4*81, Div=5*51, Dv=5*74. 



The thermometers employed were two by Geissler and two 

 by ISTegretti and Zambra. These last were verified at Kew. 

 Those by Geissler were from 20°-60° C, and from 40°-100° C, 

 and were divided into lOths ; one of the others was from 

 - 10°-40° C, also in ^ths, and the fourth from - 10°-110° C, 

 divided into half degrees (a very open scale). These were 

 carefully compared together and corrected at 20°, 45°-46°, 

 50°-51°, 56°-57°, 61°-62°, 67 c -68°, 72°-73°, and 78°-80°- 

 the temperatures at which determinations were to be made. 

 The comparisons were made in two constant-temperature 

 baths, one at 20° C, the other being the one employed for 

 heating the dilatometers. 



The constancy of temperature was in this last case obtained 

 by means of the vapour of a liquid boiling under a constant 

 pressure variable at will. The liquid in this case was a mixture 

 of alcohol and water boiling at about 82° at 760 millim. The 

 apparatus consists of two parts, the dilatometer-bath with boiler 

 and condenser, and the pressure-regulator. 



The bath is shown in fig. 3. It consists of two glass tubes, 

 one within the other, diameters 65 and 45 millim., and re- 

 spective lengths 900 and 700 millim. The longer and wider 

 of the tubes is drawn out at one end to about 15 millim. 



