MAGNUS ON THE EXPANSIVE FORCE OF STEAM. 231 
sive force therefore of the steam could scarcely fall out 0°000014 
too high from this cause, even at 105° C., the highest tempera- 
ture employed. 
The principal source of error however in these experiments is 
owing to its being impossible to expose the air thermometer and 
the vapour to absolutely the same temperature. It is on that 
account that the observations at 0°, which were made in melting 
ice, when consequently the temperature remained perfectly con- 
stant and no air thermometer was required, differ so little from 
one another. 
If the observed values be compared with Dalton’s older re- 
sults, which were founded partly on the observation of the boil- 
ing-point under the receiver of the air-pump, and partly by 
direct measurement of the expansive forces, or with the table 
ealculated by Biot from them, it will be seen that they differ 
from these in so far as the expansive forces observed by me for 
low temperatures are altogether smaller, as well as in those above 
88° C., while in the central degrees they are larger. 
Although the expansive force at 0° is supported by the coin- 
cidence of various observations, yet it differs considerably from 
the assertions of all earlier observers; and even although the 
expansive forces of the temperatures situated near 0° confirm 
the view that it must be lower than has hitherto been admitted, 
yet it is precisely this observation which is of such great im- 
portance for the establishment of a formula of interpolation, 
that it appeared to me desirable to test it likewise by another 
method. I therefore selected the former, as it had been exe- 
cuted by Gay-Lussac ; but I soon became convinced how much 
in it depended on the consideration of the capillarity if accurate 
results were to be obtained. 
A barometer tube was made use of which in its upper part 
was bent at aright angle, precisely as in Gay-Lussac’s experi- 
ment, only that it was more than 0°5 inch wide at the height 
where the surface of the mercury fell, so that in this case no 
capillary depression could take place. After some well-boiled 
water had been let into the empty space the horizontal part of 
the tube was surrounded with ice, in order that all the water 
might distil over into it. The vertical portion of the space in 
_which the vapours were was then likewise surrounded with ice 
(although this was superfluous), and then the height read off 
with a kathetometer. But here the difficulty of observing the 
