h in which it is
immersed. C.V. Boys (_Phil. Mag._, 1887, vol. 24, p. 214) showed that
these effects could be very greatly reduced by surrounding the
calorimeter with an outer tube, so that the ice inside was separated
from the ice outside by an air space which greatly reduces the free
passage of heat. The present writer has found that very good results
may be obtained by enclosing the calorimeter in a vacuum jacket (as
illustrated in fig. 3), which practically eliminates conduction and
convection. If the vacuum jacket is silvered inside, radiation also is
reduced to such an extent that, if the vacuum is really good, the
external ice bath may be dispensed with for the majority of purposes.
If the inner bulb is filled with mercury instead of water and ice, the
same arrangement answers admirably as a Favre and Silbermann
calorimeter, for measuring small quantities of heat by the expansion
of the mercury.

[Illustration: Fig. 3.]

The question has been raised by E.L. Nichols (_Phys. Rev._ vol. 8,
January 1899) whether there may not be different modifications of ice
with different densities, and different values of the latent heat of
fusion. He found for natural pond-ice a density 0.9179 and for
artificial ice 0.9161. J. Vincent (_Phil. Trans._ A. 198, p. 463) also
found a density .9160 for artificial ice, which is probably very
nearly correct. If such variations of density exist, they may
introduce some uncertainty in the absolute values of results obtained
with the ice calorimeter, and may account for some of the
discrepancies above enumerated.

S 5. The _Method of Condensation_ was first successfully applied by J.
Joly in the construction of his steam calorimeter, a full description of
which will be found in text-books. The body to be tested is placed in a
special scale-pan, suspended by a fine wire from the arm of a balance
inside an enclosure which can be filled with steam at atmospheric
pressure. The temperature of the enclosure is carefully observed before
admitting steam. The weight of steam condensed on the body gives a means
of calculating the quantity of heat required to raise it from the
atmospheric temperature up to 100 deg. C. in terms of the latent heat of
vaporization of steam at 100 deg. C. There can be no appreciable gain or
loss of heat by radiation, if the admission of the steam is sufficiently
rapid, since the walls of the enclosure are maintained at