324 On radiation, absorption, &c. 
other good absorbent and radiator, leaving a small strip of the upper 
surface bare, to exhibit the nature of the material ; the plates having 
phosphorus placed, on mica, upon them, are put upon the frame, and 
this is placed on the cover of the box : the order in which the phos- 
phorus fires, gives the same indication as in the apparatus of Ingen- 
ouz. This effect is more rapid than when cones, or rods, are used, 
especially from the lower temperature of the substance which is 
commonly used as a source of heat. These remarks do not apply, 
of course, to the forms of that apparatus in which hot sand is used. 
The second instrument to be described, is intended to show the 
common illustration of the fact that bodies have different specific 
ts... 
Theoretically, this illustration is, I think, inaccurate, but is ad- 
missible, like the last; upon this subject, I hope to be able, at a fu- 
ture time, to be more explicit ; at present, my remarks are confined 
to general illustrations. ‘That different bodies require unequal quan- 
tities of heat to raise their temperatures through the same number of 
degrees, is illustrated upon equal weights, or bulks, by subjecting 
them, when at the same temperatures, to the same source of heat, 
and proving that they require different times to arrive at the same 
temperature. This idea is a fundamental one, and cannot too early 
be inculcated upon a learner. As an illustration, I have three ves- 
sels of sheet iron, to contain equal weights of mercury, alcohol, and 
water ; these are fastened to a frame, by which they can be dippe 
into the same vessel] containing hot water. An alcohol thermom- 
eter, with a column of fluid large enough to be visible at a moderate 
distance, dips into each vessel. As the heat enters, the thermome- 
ter in the mercury rises with great rapidity, that in the alcohol more 
slowly, and that in the water lags behind both the others. Instead 
of those thermometers, if a cylinder of any metal which is a good 
eonductor, and has a low specific heat, such as copper for example, 
should, after being coated with a varnish of thickened linseed oil to 
protect the wiartaed: be introduced into each vessel, phosphorus pla- 
ced on the top, would melt and in- F; 
flame first on the metal which dip- ie 
ped into the liguid having the least 
capacity for heat. In the annexed 
cut, fig. 2, a, 6, and c, are the ves- 
sels ; d, e, f, metallic cylinders rest- 
ing in wooden, or metallic, or mica, 
