'230 PHILOSOPHICAL TRANSACTIONS. [aNNO 1777- 



more caution necessary, in boiling sucli as were ground, especially in frosty wea- 

 ther, vvliich happened to be the case in the last days of March, 1775 : it was 

 therefore thought best in the interim to try, what might be the expansion of a 

 column of quicksilver, carefully put into the tube, but not boiled therein ? 

 With this view, the standard barometer and apparatus were left out during the 

 night of the 29th, that they might acquire the same temperature, which was 

 found next morning to be 34*^4^ ; the unboiled quicksilver standing -j-i-5- of an 

 inch higher than that which had been boiled. Tlie lamps being applied to the 

 vessel, the lengthening of the unboiled column was perceived, on the whole, to 

 be more irregular, and the progressive diminution quicker, than in former ex- 

 periments ; so as to give, for the maximum of expansion, only .443 for 180°. 



On the moining of the 31st, the unboiled column, which on the preceding 

 day had been the highest, was lower than the other by near -j-s-j- of an inch, 

 the temperature of both being 31-l°. As the water acquired heat from the ap- 

 plication of the lamps, the rate of expansion diminished ; and at boiling was 

 only .405 for 180". The operation of the 30th seems to point out, in a manner 

 sufficiently conclusive, that the air contained in the unboiled quicksilver, ren- 

 dered its specific gravity less than that which had been boiled even a great while 

 before; since it required a longer column of the first, to counterbalance the 

 weight of the atmosphere. And though the vacua might possibly, at the be- 

 ginning, have been equally complete in both ; yet they could not continue long 

 so: for the air escaping gradually from the unboiled quicksilver, its elasticity m- 

 creasing with the heat, and uniting with the quicksilver vapour, must have re- 

 sisted the dilatation of the column, and rendered it less than on former occa- 

 sions : which actually appeared from experiment. This is farther confirmed by 

 the observations of the subsequent day ; for now the unboiled column was be- 

 come the :hortest, owing no doubt to more air having ascended, and rendered 

 the vacuum still more incomplete. Thus, the causes of resistance increasing, 

 the dilatation is lessened in a superior degree. 



Having found, from the first 1 sets of this class, the rate of expansion of a 

 column of quicksilver, in the tube of a barometer of the ordinary length, to 

 be progressive and not arithmetical; and that its maximum, for the 180" com- 

 preheniled between freezing and boiling, was less than had been supposed ; it 

 was thought proper to try, by means of artificial cold, whether the condensation, 

 for the 32° below freezing, followed nearly the same law ? For this purpose the 

 tin vessel, containing the ground tube, was rammed quite full of pounded ice 

 and salt, as well as the tin stand holding the iron cistern below. In this opera- 

 tion, \1 pounds of ice and 4 pounds of salt were employed, by wliich the mean 

 temperature of the mixture was reduced to -j- 4° of Fahrenheit. But before the 

 eyes of the vessel could be sufficiently freed from the composition, so as to 



