549 
• STEAM 
constant height 29.0, which was the height of the mercury 
in the barometer during the last set of observations. 
Temperature. 
Elasticity. 
32° 
0-0 
40 
o-i 
50 
0-1 
60 
0-35 
70 
0-55 
80 
0-82 
90 
1-25 
100 
1-6 
110 
2-25 
120 
30 
130 
3-95 
140 
5-15 
150 
6-72 
160 
8-65 
170 
11-05 
180 
1405 
190 
17-85 
200 
22-62 
210 
28-65 
220 
35-8 
230 
44-7 
240 
54-9 
250 
66-8 
260 
80-3 
270 
94-1 
280 
105-9 
In the memoirs of the Royal Academy of Berlin for 
1782, there is an account of some experiments made by 
Mr. Achard on the elastic force of steam, from the tem¬ 
perature 32° to 212°. They agree extremely well with those 
mentioned here, rarely differing more than two or three- 
tenths of an inch. He also examined the elasticity of the 
vapour produced from alcohol, and found, that when the 
elasticity was equal to that of the vapour of water, the tem¬ 
perature was about 35° lower. Thus, when the elasticity of 
both was measured by 28-1 inches of mercury, the tempe¬ 
rature of the watery vapour was 209°, and that of the spi¬ 
rituous vapour was 173°. When the elasticity was 18.5, 
the temperature of the water was 189.5, and that of the 
alcohol 154.6. When the elasticity was 11.05, the water 
was 168°, and the alcohol 134°.4. Observing the difference 
between the temperatures of equally elastic vapours of water 
and alcohol not to be constant, but gradually to diminish, 
in Mr. Achard’s experiments, along with the elasticity, it 
became interesting to discover whether and at what temperature 
this difference would vanish altogether. Experiments were 
accordingly made by Robison, similar to those made with 
water. They were not made with the same scrupulous care, 
nor repeated as they deserved, but they furnished rather an 
unexpected result. The following table will give the reader 
a distinct notion of them:— 
Temperature. 
Elasticity. 
32° 
o-o 
40 
0-1 
60 
0-8 
80 
0-8 
100 
3-9 
120 
6-9 
140 
12-2 
160 
21-3 
180 
34- 
200 
52-4 
220 
78-5 
240 
115- 
We say that the result was unexpected; for as the natural 
boiling point seemed by former experiments to be in all 
fluids about 120 ° or more below their boiling point in the 
ordinary pressure of the atmosphere, it was reasonable to 
expect that the temperature at which they ceased to emit 
Vol. XXIII. No. 1590. 
ENGINE. 
sensibly elastic steam would have some relation to their tem¬ 
peratures when emitting steam of any determinate elasticity. 
Now as the vapour of alcohol of elasticity 30 has its tem¬ 
perature about 36° lower than the temperature of water 
equally elastic, it was to be expected that the temperature at 
which it ceased to be sensibly affected would be several de¬ 
grees lower than 32°. It is evident, however, that this is not 
the case. But this is a point that deserves more attention, 
because it is closely connected with, the chemical relation 
between the element (if such there be) of fire and the bodies 
into whose composition it seems to enter as a constituent 
part. What is the temperature 32°, to make it peculiarly 
connected with elasticity ? It is a temperature assumed by 
us for our own conveniency, on account of the familiarity 
of water in our experiments. Ether, we know, boils in a 
temperature far below this, as appears from Dr. Cullen’s ex¬ 
periments narrated in the Essays Physical and Literary of 
Edinburgh. On the faith of former experiments, we may 
be pretty certain that it will boil in vacuo at the tempera¬ 
ture— 14°, because in the air it boils at'+ 106°. Therefore • 
we may be certain, that the steam or vapour of ether, when 
of the temperature of 32°, will be very sensibly elastic. In¬ 
deed, Mr. Lavoisier says, that if it be exposed in an exhausted 
receiver in winter, its vapour will support mercury at the 
height of 10 inches. A series of experiments on this vapour 
similar to the above would be very instructive. We even 
wish that those on alcohol were more carefully repeated. If 
we draw a curve line, of which the abscissa is the line of tem¬ 
peratures, and the ordinates are the corresponding heights of 
the mercury in these experiments on water and alcohol, we 
shall observe, that although they both sensibly coincide at 
32°, and have the abscissa for their common tangent, a very 
small error of observation may be the cause of this, and the 
curve which expresses the elasticity of spirituous vapour may 
really intersect the other, and go backwards considerably 
beyond 32°. 
This range of experiments gave rise to some curious and 
important reflections in Robison’s mind. He says, we now 
see that no particular temperature is necessary for water assum¬ 
ing the form of permanently elastic vapour; and that it is 
highly probable that it assumes this form even at the tempe¬ 
rature 32°; only its elasticity is too small to afford us any 
sensible measure. It is supposed that even ice evaporates, 
since in an experiment to this purpose by Mr. Wilson in the 
Philosophical Transactions, a piece of polished metal, co¬ 
vered with hoar-frost, became perfectly clear by exposing it 
to a dry frosty wind. 
Even mercury evaporates, or is converted into elastic va¬ 
pour, when all external pressure is removed. The dim film 
which may frequently be observed in the upper part of a 
barometer which stands near a stream of air, is found to be 
small globules of mercury sticking to the inside of the tube. 
But their elasticity is too small to occasion a sensible depres¬ 
sion of the column, even when considerably warmed by a 
candle. 
With regard to the cause why increased pressure increases 
temperature and elasticity, Robison is of opinion that “So¬ 
lution is performed by forces which act in the way of attrac¬ 
tion ; or, to express it more safely, solutions are accompanied 
by the mutual approaches of the particles of the menstruum 
and solved: all such tendencies are observed to increase by 
a diminution of distance. Hence it must follow, that air of 
double density will dissolve more than twice as much as 
water. Therefore when we suddenly rarefy saturated air 
(even though its heat should not diminish) some water must 
be let go. What may be its quantity we know not; but it 
may be more than what would now become elastic by this 
diminution of surrounding pressure.” 
Another inference which may be drawn from these experi¬ 
ments is, that Nature seems to affect a certain law in the 
dilatation of aeriform fluids by heat. They seem to be 
dilatable nearly in proportion of their present dilatation. 
For if we suppose that the vapours resemble air, in having 
their elasticity in any given temperature proportional to their 
7 A density 
