In order to obtain some idea of this temperature, I had an alcohol thermometer made, of which the graduation was carried below 32° Fahr., by degrees equal in capacity to those between 32° and 212°. When this thermometer was put into the bath of carbonic acid and ether surrounded by the air, but covered over with paper, it gave the temperature of 106° below 0°. When it was introduced into the bath under the air-pump, it sank to the temperature of 166° below 0°; or 60° below the temperature of the same bath at the pressure of one atmosphere, i. e. in the air. In this state the ether was very fluid, and the bath could be kept in good order for a quarter of an hour at a time.
As the exhaustion proceeded I observed the temperature of the bath and the corresponding pressure at certain other points, of which the following may be recorded: The external barometer was 29.4 inches:
| inch. | Fahr. | ||
when the mercury in the air- pump barometer was |
1 | the bath tempe- rature was |
-106° |
| when„the„mercury„in the„air-pump„ | 10 | the„bath„temperature„ | 112½ |
| when„the„mercury„in the„air-pump„ | 20 | the„bath„temperature„ | -121 |
| when„the„mercury„in the„air-pump„ | 22 | the„bath„temperature„ | -125 |
| when„the„mercury„in the„air-pump„ | 24 | the„bath„temperature„ | 131 |
| when„the„mercury„in the„air-pump„ | 26 | the„bath„temperature„ | -139, |
| when„the„mercury„in the„air-pump„ | 27 | the„bath„temperature„ | -146 |
| when„the„mercury„in the„air-pump„ | 28 | the„bath„temperature„ | -160, |
| when„the„mercury„in the„air-pump„ | 28.2 | the„bath„temperature„ | -166; |
but as the thermometer takes some time to acquire the temperature of the bath, and the latter was continually falling in degree; as also the alcohol thickens considerably at the lower temperature, there is no doubt that the degrees expressed are not so low as they ought to be, perhaps even by 5° or 6° in most cases.
With dry carbonic acid under the air-pump receiver I could raise the pump barometer to 29 inches when the external barometer was at 30 inches.
The arrangement by which this cooling power was combined in its effect on gases with the pressure of the pumps, was very simple in principle. An air-pump receiver open at the top was employed; the brass plate which closed the aperture had a small brass tube about 6 inches long, passing through it air-tight by means of a stuffing-box, so as to move easily up and
