PROFESSOR KOPP ON THE SPECIFIC HEAT OF SOLID BODIES. 
87 
cipally used, which I will here mention. The use of the mercury-bath makes it possible 
readily to produce, and maintain for any adequate length of time, any temperature de- 
sirable in such experiments. The mercury-bath* shares with the air-bath the advantage 
that, to the substance heated in it (in this case the tube and contents), nothing adheres 
when it is removed which might influence the thermal effect in the calorimeter. It 
has over the air-bath the advantage, that any body placed in it takes the tempera- 
ture of the surrounding medium much more quickly through its entire mass. The 
communication of heat to the solid substance is materially promoted by the circulation 
of the liquid between its particles ; the time necessary for the entire contents of the 
glass to become equally heated is a very short onef. Moreover this very circulation 
of the liquid between the particles of the solid ensures a quicker and more uniform 
transmission of the heat of the contents of the glass to the water of the calorimeter ; 
the maximum temperature of this water is soon attained^, although the transmission 
of the excess of temperature must take place through the sides of the glass. 
* In 1848 I already used such a one for heating liquids enclosed in glass tubes, in determining their specific 
heats (Poggendorff’s ‘ Annalen,’ vol. lxxv. p. 98). 
t In experiments on the scale on which I made them, when the mercury-bath had once been raised to the 
requisite temperature, it only required ten minutes’ immersion of the glass in the bath to impart to it the tem- 
perature of the bath. A more prolonged heating was found to be useless in all cases in which I tried it. In the 
experiments to be subsequently described, the heating was continued about ten minutes ; in most cases less would 
have been sufficient. In Regnatjlt’s experiments (Ann. de Chim. et de Phys. [2] vol. lxxiii. p. 22), in which the 
substance (in much larger quantities it is true) was heated in a space nearly surrounded by steam, a thermo- 
meter placed in the substance indicated after about two hours an almost constant position (always one or two 
degrees lower than the temperature of the steam) ; and then it was found convenient to continue this heating for 
at least an hour, in order to see that the temperature did not change, and to be certain that the substance had 
the temperature indicated by the thermometer throughout its entire mass. In Neumann’s experiments, the space 
in which the substance to be heated is contained is smaller and more completely surrounded by vapour. The 
time necessary for heating the substance uniformly must be smaller, and the temperature must be nearer that of 
the surrounding vapour. According to Pape (Poggendorff’s ‘ Annalen,’ vol. cxx. p. 352), a thermometer placed 
in the above space, if surrounded by steam for forty-five to sixty minutes, gives exactly the temperature of 
this steam. 
+ In several experiments I determined the time which elapsed between immersing the glass with contents 
in the water of the calorimeter and its attaining a maximum. Under the circumstances, which I subsequently 
give more specially, and which, as far as possible, were maintained in all experiments, this time was always less 
than two minutes, if the liquid could circulate between compact pieces of the solid substance. What I have said 
above justifies, I think, my not having made, in experiments with such substances, a correction for the loss of 
heat which the calorimeter experiences between the moment of immersing the glass and the establishment of a 
maximum temperature. In substances which form a fine powder or a porous mass, or in general in cases in which 
the circulation stagnates, the maximum temperature is more slowly attained, the above loss of heat is more con- 
siderable, and the numbers for the specific heats are then somewhat too small. I shall recur to this again in 
enumerating the experiments in § 41 with chromium, and in § 52 with chloride of chromium. In a few cases 
I have endeavoured to diminish this error, and to promote the circulation of the liquid by pressing the porous 
substance into small disks. I must leave it as an open question whether more accurate results would not be 
obtained for such substances if they were formed by means of a suitable cement into compact masses, and then 
the thermal action of the cement thus added taken into account. 
MDCCCLXV. 
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