388 IVIE. W. HOPKIXS OX THE COXSTEECTIOX OE A XEW CALOEDIETER 
Thus I could observe with great accuracy both the time during which radiation took 
place, and the increase of temperature acquu'ed in that time by the calorimeter, and 
these are manifestly the two principal quantities to be determined by observation in the 
experiments. There are several possible sources of error, however, which require 
examination. 
7. The first obvious consideration is that the water contained in the vessel A of the 
calorimeter does not, during the experiment, merely receive heat from the heated 
radiating surface ; it will also, at each instant, be gaining or losing heat by the exposure 
of its extern-al surface to the atmosphere, whenever its temperature difiem from the 
surrounding temperature. I endeavoured to ob'viate this in the first place by making the 
vessel A of tin, the external surface of which, exposed to the atmosphere, was _poIis}i€d, 
while that exposed to the radiation from the heated disc, as already stated, was covered 
with lamp-black. The external surface was therefore a bad radiator of heat from within 
to the external ah’, and a bad absorbent of heat from wittibut, and consequently helped 
to reduce the influence of the external air. Further, it is manifest that no eflect could 
be produced on the experimental results, prowded the mean temperature indicated by 
the thermometer of the calorimeter during an experiment were equal to the tempera- 
ture of the external air. This condition was more or less approximately satisfied in the 
actual experiments, the results of which have been relied upon ; but when not satisfied, 
it was easy to ascertain and apply the requisite correction when of sufficient amount to 
be sensible. To ascertain this correction, I made the following experiment. The whole 
instrument (E) was placed on an iron disc; it rested on its three feet {t). The vessel 
(A) of the calorimeter and the iron disc were immersed in water, of which the tempera- 
ture was about 6° below that of the surrounding air. When the temperatmn indicated 
by the sensitive thermometer of the calorimeter had become perfectly stationary, the 
instrument and iron disc were taken out of the water, rapidly cleared of the external 
moisture, and the instrument replaced on the iron disc in the air. The under blackened 
surface of the vessel A was now exposed only to the surface of the iron disc, which was 
of the same temperature as the distilled water in A, and could not, therefore, afiect its 
temperature. At the same time the external polished surface of A was exposed to the 
external air, of which the temperature was about 5° higher than that of the water of 
the calorimeter. The result, however, w^as that the thermometer of the calorimeter 
remained sensibly quite stationary for a considerably longer time than that requried for 
any of the experiments, thus proving that the corrections in my actual experiments due 
to this cause were of insensible amount. 
8. When a thin metal disc was used for the radiating surface, a small orifice was made 
at its centre, as above stated, just large enough to admit the bulb of a thermometer used 
to determine the temperature of the mercury beneath the disc. Over the area of this 
orifice the radiation took place from the surface of mercury, and not from that of the 
disc. Also, when a thicker disc of a mineral substance was used, the superficial hollow 
tor the coiled bulb of the thermometer was filled with mercury, fi'om the surface of 
