VOL. LXVII.] PHILOSOPHICAL TRANSACTIONS. 'I'lQ 



total expansion between freezing and boiling; for the 20°, between 112'^ and 

 132% it was .059; and for the last 20", between 192° and 212°, it was .046. 

 In these experiments, the tube being wholly covered with boiling water, no con- 

 densation of vapour took place in the vacuum ; and therefore no particles of 

 quicksilver were seen adhering to the upper part of the tube. When the water 

 boiled, the resistance of the vapour was greater than in the preceding set, and 

 the total expansion less. These 2 results serve strongly to confirm each other: 

 it is however the last that furnishes the data for constructing the table of equa- 

 tion depending on the heat of the quicksilver in the barometer. 



Finding, from the comparison of these two sets of experiments with each other, 

 that the maximum and rate of expansion seemed to vary with the length of the 

 vacuum above the quicksilver. Dr. Blagden advised to try what might be the result, 

 when the vacuum was much longer than in the common barometer. The 3d set 

 of experiments of this class was therefore made with a tube somewhat narrower in 

 the bore than the former, and whose vacuum was 14-1- inches in length, of which 

 11-^ reached above the top of the vessel. The mean of 3 observations gave 

 .5443 for the total expansion on 180° ; that for the first 20° was .067 ; for the 

 20° in the middle of the scale .058 ; and for the uppermost 20°, it was .065 : 

 whence the mean rate for every 20°, is nearly .0605.* In this set, the conden- 

 sation in the vacuum of the tube was particularly attended to : it began, as in 

 those of the first set, immediately above the surface of the boiling water, which 

 was always kept an inch or 2 above the top of the column : the lowermost glo- 

 bules were very small, increasing gradually till they got without the lid of the 

 vessel, where they were the largest ; thence they diminished uniformly upwards, 

 and disappeared entirely 3 or 4 inches below the top of the tube. Though the 

 rate for the middlemost 20°, in these last experiments, be below the mean, 

 probably from some inaccuracy in observation ; yet, being compared with the 

 former sets, they still serve to corroborate each other : for in these with tlie long 

 tube, the vacuum seems to have been either completely maintained, or nearly 

 so ; and we accordmgly find the maximum of expansion increased, and its rate 

 rendered nearly uniform. 



In the course of the preceding experiments, from accidents of various kinds, 

 it was often necessary to reboil the quicksilver ; and in that operation many tubes 

 were broken. The frequent removal of the soc kfrom the bottom of the vessel, 

 in order to get others ground for it, became at last very troublesome ; and made 



* Mr. Cavendish, who assisted in the first part of the experiments with the open tube, informed 

 me, that, in those made by his father Lord Charles, the difference between the expansion of quick- 

 silver and glass, from 180° of heat, was .4-69. If to tliis we add Mr. Smeaton's dUatationof ^lass, 

 the total expansion of 30 inches of quicksilver will be .544, which agrees with the experiments in 

 the long tube, and gives a rate of only .003022 for each degree. — Orig. 



