PYRONOMICS. 107 
transmitted the heat of all equally well; plates of ice only that of the 
Locatelli Jamp. For all other sources the power of transmitting was 
zero. 
That heat rays are capable of refraction like those of light, may be shown 
by the apparatus represented in pl. 19, fig.41. Upon a stand is placed a 
prism of rock salt, and at some distance the Locatelli lamp. The direction 
is now observed in which the rays of light emerge from the prism with the 
least deviation from their original direction, and the thermo-electric pile 
placed in it: the needle will become immediately deflected. The same will 
be the case if, for the Locatelli lamp, the platinum spiral, the cube of hot 
water, &c., be substituted. The deflection ceases immediately on slightly 
moving the pile, whence it follows that the rays from the different sources 
are refracted by the rock salt. 
In this great difference in the transmitting power of diathermanous bedies 
the question suggests itself, whether in the athermanous bodies the power 
of absorption and diffusion be not different. Melloni has instituted the 
investigations necessary to answer this question. He cut out disks of equal 
diameter from the same copper plate, blackened them on one side, and 
coated them on the other with the substance whose power of absorption was 
to be ascertained. He then introduced the plates, one after the other, into 
the apparatus, so that the blackened side was directed towards the pile, and 
the coated side towards the source of heat. This side became heated by 
absorption, and this heat, being communicated to the opposite side, was 
brought to bear upon the pile. He thus discovered a great difference both 
in the absorbing power of the bodies themselves, as also in respect to the 
different sources of heat. Lamp-black exhibited the maximum power of 
absorption, only 13% of which was exhibited by a bright polished surface. 
Melloni and Forbes have also indicated a polarization of heat rays, 
by processes similar te those by which the same change is produced in 
light. 
Dulong and Petit have instituted the most accurate experiments upon the 
laws of cooling by means of the apparatus represented in fig.42. Here ais 
a copper vessel filled with water kept at a uniform temperature; 0 is 
a balloon of copper plate, blackened internally and sunk in the water ; it is 
sustained by the frame c. Upon the broad ground edge of the balloon 
is placed a level plate, d, of thick glass, and upon this (like a receiver on the 
plate of an air-pump) a broad glass tube, e. This is provided above with a 
cock, and is connected by a leaden tube, g, with an air-pump, of which 
the figure represents only the plate h. The tube k is filled with chloride of 
calcium, which serves to dry the gas coming from the gasometer J, in case 
experiments are to be made upon cooling in different gases. The bodies 
whose cooling is to be observed in this apparatus are large thermometers 
with spherical bulbs, fastened by a cock in the glass plate d, and capable of 
being raised with it. When such a thermometer has been heated to the 
proper temperature, it is quickly introduced into the balloon, the tube e placed 
over it, and the air pumped out. The depression of the mercury is to be 
observed with the assistance of a good watch. 
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