380 ME. W. HOPKIJs^S OX THE COXSTEUCTIOX OE A XEW CALOEIMETEE 
was firmly fixed in a vessel containing water in which the globe could he entii-ely 
immersed. An orifice existed in the upper side of the globe, through which a thermo- 
meter could be inserted and anunged so that the centre of its spherical bulb might 
coincide with the centre of the globe, and the orifice be hermetically closed. In makin" 
the experiment, the mercury of the therTuometer was heated to any proposed tempera- 
ture, arrd then rapidly placed in its position in the globe, where the rate of its cooling 
was determined by observing at regular interwals of time the temperature indicated by 
the graduated stem of the thermometer. The interior surface of the copper globe was 
blackened to prevent as much as possible all reflexion of heat, and was kept at a con- 
stant temperature by means of the water in which the globe was immer-sed. The expe- 
riments could thus be made at any temperature of the medium immediately surroimding 
the bulb, and also at any pressure of that mediirm, by withdravring a portion of the 
atmospheric air, if that were the medium ; or by withdrawing it enthely, and then filling 
the globe with any proposed gas at any assigned pressure. The rate at which the tem- 
perature of the mercury decreased gave a measure of the rate of its cooling, and conse- 
quently of the quantity of heat which emanated fi’om the sruface of the brrlb in a given 
time under given conditions. 
This apparatus was admirably adapted for experimenting with those substances of 
which the bulb of a thermometer might be formed, or with which the surface of the 
bulb could be uniformly and thinly coated ; but was manifestly not applicable to deter- 
mine the radiating power of the surfaces of mineral and many other substances. Tlie 
simpler instrument which I have employed is equally applicable to all such surfaces, 
while it allows the extent of the radiating surface to be ascertained with greater 
accuracy than that of the bulb of the thermometer from which the radiation took place 
in the apparatus just described, and is so far better adapted for finding the absolute 
quantity of heat which radiates from a given area of a given surface in a unit of 
time. It does not, however, afibrd the means of determining the quantity of heat wiiich 
radiates from a proposed surface in a vacuum, or wiien surrounded by different gases at 
various temperatures and pressures. To determine the laws of cooling m such cases, it 
was necessary to have recourse to some more complicated apparatus like that made use 
of by Dulong and Petit. 
2. The laws of cooling of a body surrounded by air or gas, or when placed in a 
vacuum, as established by these able experimenters, are embodied in the following 
formula, — 
Q=A«V-I)-fBp^ri, (I.) 
w^here 
Q= quantity of heat which escapes from the radiating surface in a given time; 
«=:1'0077, a numerical quantity wdiich is the same for all radiating surfaces, and 
surrounding media ; 
^=the uniform temperature of the surrounding vacuum or medium, expressed in 
Centigrade degrees ; 
