350 
cannot easily escape, the arrangement in 
question is virtually a furnace whose ef- 
ficieney accelerates with rise of tempera- 
ture or increase of terrestrial depth. 
PIEZOMETRY. 
It is not feasible to make much progress 
in pyrometry without feeling the need of a 
corresponding development in high pressure 
measurement. This has already appeared 
in the preceding parts of my address. It 
will not be expedient to look into the his- 
tory of the subject as comprehensively as I 
did in the case of pyrometry, partly because 
the literature is more diffuse, and partly be- 
cause the real development of piezometry is 
of recent date and virtually begins with 
pressures of the order of several thousand 
atmospheres. So understood, although we 
gladly pay homage to Oersted, to Regnault, 
to Grassi and many others, our historical 
summary may be abridged. 
As is often the case in physics, the great 
advances in the subject are associated with 
the name of one man; for, though many 
able investigators have contributed effect- 
ively to the progress of piezometry, the 
overshadowing importance of the results of 
Amagat have superseded all researches co- 
extensive with his own. For over twenty 
years Amagat has been laboring on this defi- 
nitely circumscribed subject. Year after 
year his prolific experimental ingenuity has 
put forth results, each of which in its turn 
constituted the highest attainment in accu- 
racy and the greatest breadth of scope 
which high pressure measurement had 
reached at the time. It is impossible to 
give any adequate view of this sustained 
labor in an address. The subject is highly 
specialized and demands special treatment ; 
but we owe to Amagat the bulk of our 
knowledge of the properties of a gas re- 
garded not as an ideal fluid, but as a phys- 
ical body ; some of the most far-reaching 
results in the thermo-dynamics of liquids, 
SCIENCE. 
[N.S. Von. VI. No. 140. 
and some of the best data in the elastics of 
solids. 
Amagat investigated gases within an in- 
terval of pressure which at times reached 
4,000 atmospheres, with a view to interpret- 
ing their divergence from the laws of ideal 
gaseify. Indeed, we may note in passing that 
just as the advanced astronomy of the day is 
being enriched with unexpected discoveries 
from a discussion of mere errors of observa- 
tion, so refined physical measurement gleans 
new harvests in carefully tracing out the 
all but rigorous sufficiency of established 
laws. The product of pressure and volume, 
nearly constant in the ordinary isothermal 
behavior of gas, shows, under higher pres- 
sures, a well-marked passage through a 
minimum in the case of all gases except hy- 
drogen. Hence, below a certain definite 
pressure, varying with the character of the 
body (say 40 atm.), gases are more com- 
pressible than Boyle’s law asserts, and 
above this pressure they are continually less 
compressible and begin to resemble hydro- 
gen in this respect. The sharpness of the 
minimum diminishes as temperature in- 
creases, and probably ultimately vanishes. 
Cailletet, it is true, had undertaken a study 
of the same subject simultaneously, but his 
results were not in the same degree correct. 
Again, the coefficient of expansion of gases, 
considered in its isopiestic behavior for tem- 
peratures not too far above the critical 
point, increases with pressure to a maxi- 
mum, which seems to occur at the same 
pressure for which the volume-pressure 
product isaminimum. This thermal max- 
imum also decreases with temperature and 
finally vanishes. 'To specify the conditions 
further than this would be to exceed the 
limits beyond which verbal statement ceases 
to be lucid. The value of Amagat’s work, 
however, is not merely the formulation of 
such general laws for gases as a whole, but 
rather the investigation of sharp and spe- 
cific results for each gas individually. Thus, 
