94 THE ANTARCTIC MANUAL, 
in common use. Yet it is one which is very simple in its practice 
and accurate in its results. In the course of an investigation into 
the boiling-points of aqueous solutions, the writer had frequent occa- 
sion to determizte the boiling-point of pure water, or, more properly, 
the temperature of saturated steam at the barometric pressure of the 
moment; and he has arrived at a form of apparatus which gives this 
with great exactness and constancy, no matter how delicate the 
thermometer used may be. 
The instrument generally used for fixing or 
verifying the boiling-point of a thermometer is 
that of Regnault, which is described and figured 
in nearly every treatise on physics, eg. Balfour 
Stewart’s ‘ Lessons in Elementary Physics, 1870, 
p. 151. For thermometers having their boiling- 
point not too near the bulb, this instrument gives 
fairly trustworthy results. There are, however, 
several disadvantages. First, the stem of the 
thermometer is not wholly immersed in the steam, 
and there is uncertainty about the tempera- 
ture of the mercury in the portion of the stem 
inside the cork and projecting beyond it. It is 
disadvantageous and unnecessary to make the 
steam space of the apparatus of metal, and to 
dispose it in the form of an inner space and sur- 
rounding jacket. The want of transparency of 
the metal is an obvious disadvantage, and the 
jacketing is unnecessary, because the latent heat 
of steam is so great, that, with the supply of it 
which the boiler of Regnault’s apparatus can 
furnish with any efficient lamp, a steam vessel of 
single envelope, under ordinary conditions, cannot 
be cooled by even a fraction of a degree below 
the temperature of saturated steam correspond- 
ing to the existing barometric pressure. Any 
cooling of the outside surface of the envelope is stopped at once and 
perfectly by the film of water continually descending along its inner 
surface, while the inner surface of the film is freely exposed to an 
abundant supply of saturated steam. 
The object of the experiment isto produce an atmosphere of saturated 
steam, the tension of which is equal to the pressure of the atmosphere, 
and to have a trustworthy thermometer so immersed in it that it 
assumes, and retains without variation, the temperature of the steam. 
