64
THE NATURE AND
[sect. II.
To determine the pressure at higher temperatures with the apparatus, the end K
of the tube M N K was inserted at E; and as the temperature increased, the
pressure of the steam in the cistern L caused the mercury to ascend, and con
sequently afforded a means of measuring the amount of expansive force.
The objection to this mode of trial is, that the temperature of the mercury must
be continually changing during the trial, and steam must be either condensing or
generating on its surface during the time of observation. At each observation the
temperature of the whole of the apparatus ought to be the same, and then the
column exhibiting the pressure ought to be reduced to its equivalent at the mean
temperature. The only observation where these circumstances would have place
was that which appears to have been made when the thermometer was at 42°;
then the column in the syphon was 29*7, and the barometer stood at 29’84 : the
difference is the force of steam at 42°, and is 014 inches. By cooling down to 32°
the force v r as not perceptibly different, and we know r from later trials that this is
nearly correct. Professor Robison, however, seems to have thought it w r as neces
sary to have the force 0 at 32°. 1
Robison’s Experiments on the Force of Steam.
Temperature of
the steam.
Force of steam in inches of mercury.
Temperature of
the steam.
Force of steam in inches of mercury.
By Dr. Robison's
experiments.
By our Rule,
page 59.
By Dr. Robison’s
experiments.
By our Rule,
page 59.
32°
0-0
0-172
160°
8-65
10-05
40
0-1
0-245
170
11-05
12-6
50
0-2
0-37
180
14-05
15-67
60
0-35
0-55
190
17-85
19-35
70
0-55
0-78
200
22-62
23-71
80
0-82
1-106
210
28-68
28-86
90
1*18
1-53
220
35-8
34-92
100
v 1-6
2-08
230
44-5
42-0
110
2-25
2-79
240
54-9
50-24
120
3-0
368
250
66-8
59-79
130
3-95
4-81
260
80-3
70-8
140
5-15
6-21
270
94-1
83-45
150
6-72
7-94
280
105-9
97-92
If the elastic force *14, from which Robison began to register, had been added to
all the experiments below 212°, as it ought to have been, they would have agreed
extremely near with the results of later experiments. The experiments made by
Achard seldom vary more than a degree or two from those in the above table.
96. Mr. Dalton’s inquiries w r ere conducted by a different method. He took a
1 Meehan. Phil. voi. ii. p. 36.