in the Standard Points of Thermometers. 125 



I now come to the concluding experiment with these in- 

 struments, and, it appears to me most interesting and ano- 

 malous. The four tubes being placed in pounded ice, the 

 columns stood at the points indicated in the last column of 

 the Table ; in this situation the tops of the tubes were broken 

 off, so as to admit the free pressure of the air, and instantly 

 the thermometers fell, in the order of their numbers, '54, '43, 

 •40, '35 of a degree, now indicating on their scales + *30, 

 + -40, + -50, + -35. The remarkable features shewn by this 

 experiment are ; first, that the two thermometers sealed with 

 vacuum, and the two having air over their columns, should 

 have risen nearly equally, when two had their bulbs pressed 

 with the whole force of an atmosphere, while the other two 

 had no pressure externally, farther than that caused from 

 changes in the pressure of the atmosphere. Next, that on 

 being opened, those with air over them should have started 

 down nearly as much as those with a vacuum ; and on all 

 these appears a permanent change from three to five -tenths 

 of a degree. I confess that I am very much at a loss to ac- 

 count for these singular changes ; atmospheric pressure on 

 the bulbs would account for the change in those sealed with 

 a vacuum ; for we can easily suppose that a permanent form 

 had been taken from long exposure to that pressure by the 

 glass forming the bulbs : besides this permanent form, there 

 appears to have been a spring inwards, which instantly 

 sprung out on removal of the pressure by the admission of air 

 over the mercury ; but the same reasoning will not apply to 

 the thermometers having air over the mercury ; and before 

 I attempt to make any suggestions as to the cause of these 

 changes, I propose to institute the following experiments. 

 Having had three thermometers blown and filled with mer- 

 cury, I shall make one with a perfect vacuum over the mer- 

 cury, the next with air over it, and the third with air con- 

 densed over it ; and, noting the changes that may go on in 

 these, I hope to be able to assign a cause or causes for the 

 change. It is argued by some continental writers on this 

 subject, that the reason why we do not perceive any change 

 in the freezing point in spirit-thermometers is from the great 

 expansion of spirit above mercury, volume for volume, there- 



