July 13,1872.] 
THE PHARMACEUTICAL JOURNAL AND TRANSACTIONS. 
27 
:sure it is almost completely resolved, "with, evolution of 
hut little gas, into hydrocarbons which remain liquid at 
the ordinary temperature, and consist principally of 
olefines. 
(7b be continued.) 
THE MEASUREMENT OF TEMPERATURE BY 
ELECTRICITY. * 
BY C. WILLIAM SIEMENS, D.C.L., I’.R.S., M.U.I. 
The truth recently revealed to us by one of the younger 
•branches of physical science has divested heat and elec¬ 
tricity of their mysterious character, and has taught us 
•to regard them simply as “ modes of motion.” 
Light also has been shown to be identical in its nature 
•with heat, and the only remaining physical agency, 
‘‘chemical affinity,” has been recognized as a force dif¬ 
fering only in “quality of action” from the others. 
According to these views, force, in whichever type of 
•action it presents itself, is as indestructible as matter 
itself, and is therefore capable of being stored up and 
measured with the same certainty of result. We have a 
uni t of force or the foot lb., and a unit of heat, or the 
heat necessary to raise the temperature of 1 lb. of water 
through one degree Fahr., and it has been already 
proved that 722 units of force are the equivalent value 
•of one unit of heat. Again, the chemical force residing 
in 1 lb. of pure coal is equal to about 14,000 heat units, 
or 14,000 x 772 = 10,808,000 ft. lb. = 4825 tons lifted 
one foot high. 
Questions regarding the quantitative effects of heat 
present themselves, however, much less frequently for 
our consideration than questions regarding its intensity, 
.upon which depends the nature of the phenomena sur¬ 
rounding us at every step, both in science and in ordi¬ 
nary life. The instrument at our command for deter¬ 
mining moderate intensities or temperatures, the mer- 
*cury thermometer, leaves little to be desired for ordinary 
use ; but when we ascend in the scale of intensity, we 
;SOon approach a point when mercury boils, and from 
dhat point upward we are left without a reliable guide. 
The result is, that we find in scientific books on che¬ 
mical processes, statements to the effect that such or 
.such a reaction takes place at a dull red heat, such 
.another at a bright red , a cherry red , a blood red , or a white 
beat—expressions which remind one rather of tho days 
of alchemy than of chemical science of the present day. 
There are pyrometers, it is true, but these are either of 
a complex nature, or little reliance can be placed on 
them. 
An instrument which fulfils, in principle, all the con- 
•ditions essentially necessary in thermometry, and is, at 
dhe same time, the very first instrument that was ever 
proposed for measuring temperatures, is the air thermo¬ 
meter of Galileo. Theoretically, the expansion of a per¬ 
manent gas at constant pressure is the most perfect 
index of temperature, it being the degree of energy of the 
.atomical motion in an elastic fluid which determines its 
wolume and constitutes at the same time its tempera¬ 
ture. 
This air thermometer consists simply of a bulb of glass 
with a long tubular stem, open to the atmosphere at its 
•extremity. If the bulb be heated,—by dipping it, for 
instance, in boiling water, —and put into a holder with 
the hollow stem reaching downward into a cup of mer- 
.eury, the air within the bulb will no longer commu¬ 
nicate with the atmosphere, because the mercury is 
interposed. If the air within the bulb be now 
cooled by the external application of iced water, its 
heat motion will diminish, and its volume would be re¬ 
duced proportionally if the external atmosphere could 
•enter freely to fill up the vacancy thus created. . But 
inasmuch as the external air cannot enter, a induction of 
* Abstracted from a Paper read at the Royal Institution, 
.March 1st, 1872. 
pressure will take place, which, according to the law 
of elasticity by Boyle, must be proportionate to the re¬ 
duction of volume at constant pressure. The difference 
of pressure thus created between the bulb and the ex¬ 
ternal atmosphere will be balanced by the column of 
mercury rising up into the tube, and the elevation to 
which the mercury attains is a true index of the tempe¬ 
rature to which the air in the bulb had been previously 
heated. This is true with regard to all temperatures, 
from the lowest to the highest, and the instrument may 
be termed a universal thermometer. If the bulb could 
be cooled down to 273° Centigrade below the zero point, 
it would follow by the law of Charles that the elastic 
pressure of air would be reduced to nothing, that is to 
say, the motion of the particles of air, which we call 
heat, would have ceased, and we should have reached 
the point of absolute zero, a point which has been theo¬ 
retically established also by other means. 
Practically, such an instrument would be most incon¬ 
venient ; its indications would have to be corrected by 
calculation for barometrical variations ; the capacity of 
the descending tube, which contains air not subjected to 
variation of temperature, would have to be taken into 
account, and no reliable observations could be arrived 
at, without taking special precautions, such as are only 
within reach of the experimental physicist. 
As tho result of occasional experimental research, 
spread over many years, an instrument lias been con¬ 
structed by Mr. Siemens, by which he aims to accom¬ 
plish a double purpose, the measurement of high tem¬ 
peratures and the measurement with accuracy of tempe¬ 
ratures of inaccessible or distant places. If “electrical 
resistance in conductors” be substituted for “ expansion 
of gases,” this instrument presents many points of ana¬ 
logy with the air thermometer. Both these effects are 
functions of temperature, increasing with the tempera¬ 
ture according to progressive laws ; in the case of gases 
called the “law of Charles,” in that of conductors “the 
law of increase of electrical resistance with tempera¬ 
ture.” The latter law, which is of recent origin, had 
been already partially developed by Arndsen, Swanberg, 
Lenz, and Werner Siemens, when in 1860 the authors 
attention was directed towards an application of it to 
the measurement of temperatures of places inaccessible 
to the ordinary thermometer. 
The conductivity of a wire of platinum or other metal 
is greatly influenced by temperature. If the current of 
a galvanic battery be directed through two branches of 
equal resistance, each branch consisting of a free spiral 
wire of platinum and one of the coils of a differential 
galvanometer, no deflection of the needle occurs as long 
is the temperature of both branch circuits is the same. 
But if a flame or even a warm hand be brought in con- 
;act with one of the platinum coils, the electric resistance 
)f the heated wire is increased, and the current through 
he cooler circuit preponderating, the needle^on the face 
)f the galvanometer is immediately deflecteu. A small 
nstrument, constructed by winding thin insulated wire 
>f any pure metal upon two small cylindrical pieces of 
ivood, and enclosing the spirals in. silver casings, taking 
;are that the extremities of the spiral wires are soldered 
;o thicker insulated wires leading to the battery and to 
he galvanometer respectively, might be employed 'with 
idvantage in physiological research.. If one case, which 
reed not be larger than a small pencil-case, be placed in 
he part the temperature of which is required to be 
neasured, and the other placed in a convenient me- 
iium, such as water, when, by the abstraction or addi- 
;ion of heat, the needle of the galvanometer is 
brought to an undcflected state, the temperature of 
3 ucli~medium will be identical with that sought, and 
may be taken by a thermometer in the usual way, 
and the temperature may be read from time to time 
without disturbing the patient. It will be evident that 
such an instrument, modified to meet particular circum¬ 
stances, might be used for measuring the temperature of 
