352 
pressure gauges which have come into use. 
As far as 1,000 atmospheres, the Bourdon 
gauge, if well constructed, does good ser- 
vice, though in a somewhat rough way. 
The corrected nitrogen closed manometer is 
more accurate for a smaller range. <A 
theoretically simpler pressure gauge was 
proposed by Tait and Cailletet. In this 
case a straight cylindrical elastic tube 
under internal or external pressure is sub- 
stituted for the Bourdon tube and the ex- 
pansion or compression is directly measured. 
With due precautions against changes of 
temperature and the choice of a solid of 
constant bulk modulus and rigidity, this 
gauge can be used as far as about 2,000 
atmospheres with convenience. 
Above 2,000 atmospheres Amagat’s Bra- 
mah press manometer, already mentioned, 
is the only reliable gauge, and, though some- 
what cumbersome, has the advantage of 
giving absolute results. However, a gauge 
based on the change of electric resistance 
of mercury with pressure, a constant now 
fairly well known from Palmer’s measure- 
ments, will, in my judgment, do good service 
for pressures which exceed even the range 
of the manometer. With regard to methods 
for producing high pressures, the force pump, 
with a small steel plunger and the screw 
advancing bodily into a closed barrel filled 
with a liquid, have not yet been superseded. 
The efficiency of such apparatus depends 
essentially on the means used for obviating 
leakage. These must, of course, be very 
perfect. 
Amagat’s work with liquids was extended 
chiefly in the direction of high pressures 
Experiments have since been made by 
others throughout higher temperatures 
(310°) and, of course, a smaller range of 
pressures (500 atm.). Leaving out the less 
perspicuous results, I will here merely al- 
lude to the probable existence of a remark- 
able law which these researches have de- 
veloped. Dupré (1869) and afterwards 
SCIENCE. 
[N.S. Vou. VI. No. 140. 
Lévy (1878), reasoning from thermo-dy- 
namic premises, were the first to suspect that 
the isometries or lines of equal volume of 
liquids are straight. In other words, if 
there is to be no change of volume then 
pressure increments must vary proportion- 
ately to the temperature increments (p= 
a0—b), a result which implies that the in- 
ternal pressure of a body kept at constant 
volume is proportional to its temperature. 
Lévy’s deduction was, however, declared to 
be theoretically unwarrantable by Clausius, 
Boltzmann and others. Sometime after, 
the same law reappeared in experimental 
form in a series of brilliant researches on 
critical temperatures due to Ramsay and 
Young. Fitzgerald, reasoning from Ram- 
say and Young’s results, then proved that 
for such liquids as possessed straight iso- 
metrics specific heat is a temperature func- 
tion only, and energy and entropy are each 
expressible as the sum of a mere tempera- 
ture function and a mere volume function. 
This is curiously like the position from 
which Dupré and Lévy started. Ramsay 
and Young’s work, however, applied spe- 
cifically to vapors, and for high tempera- 
tures (200°) their pressures did not exceed 
60 atmospheres. The law has since been 
tested for liquids as far as 1500 atmospheres 
and over 200° conjointly, and found in 
reasonable accordance with experiment. 
Hence we infer that if the thermo-dynamic 
change of a body is such that volume re- 
mains constant, pressure and temperature ~ 
will vary linearly with each other, the in- 
ecrements being about 0.1° C. per atmos- 
phere. Now, although any law relating to 
the liquid state would be welcome, these 
volume isometries are particularly so. In 
the geology of the earth’s crust, for in- 
stance, they would, in a great measure, de- 
termine the conditions of possible convec- 
tion ; and it is curious to note that, from the 
known values of bulk modulus and of the 
expansion of solid glass, the isometrics 
