CATALOGUE. — HEAT, RADIATION.

407

found that a blackened wire was sooner heated and cooledthan a wire not coloured.

Hutton's dissertation on light, heat and fire. 8.Ace. Ed. tr. IV. 11. 7. Head 1794.

Calls radiant heat obscure light; observes the greater heatof the red rays, and says that a blackened theimometer ismost sensible to the effect of obscure light, as well as tothat of visible light.

# lIersohel on the heat of the prismatic rays,on the invisible rays of the sun, and onthe solar and terrestrial rays that occasionheat. Pli. tr., 1800. ‘255, 284, 293, 437.Nich. IV. 330, 360. V. Oy. l»li. M. VU.VU1. Gilb. VU. 137.

■With many experiments on the transmission of heat.Leslie’s observations on light and heat.Nidi. IV. 344, 416. Gilb. X . 88.

* Leslie on heat.

Benzenbcrg’s remarks on Leslie. Gilb. X .356.

llermsfadt on the effect of heat on differentcolours. A. Berl. 1801. S3.

Sclnnidtiniiller on the heat communicatedto wood by the sun’s rays. Gilb. XIV.306.

Bum ford’s experiments on radiant heat. Ph.tr. 1804. 77. B. Soc. Phil. n. 87. Gilb.XVII. 33, 218.

On the effects of colours, and on the nature of thesurface.

Bbekmann’s prize essay on the heating ofbodies in the solar rays. Note. Gilb. XVII.122 .

On the velocity of radiant heat. Ph. M.XIX. 309-

l’arker's was a double convex lens, three feet in diameter,3 inches thick in the middle: it weighed si 2 pounds. Itsaperture when set was 32^ inches ; its focal length 0 feeta inches: the focal length was generally shortened by asmaller lens. The most refractory substance fused was acornelian, which required 75" for its fusion; a crystalpebble was fused in 0"; a piece of white agate in 3o".Cavallo. The linger might be placed in the cone of rays

within an inch of the focus, without inconvenience,lmison’s elements. 1. 371. But tins remark appears torequire confirmation : if it were accurate, we might expectthe smallest imperfection in the focal adjustment of the eyeto cause a great difference in die apparent brilliancy of anobject; which is not the fact: indeed Count Rumford ’slate experiments appear wholly to confute it.

Leslie discovered, by experiments made in 1802 , that theheat emitted by radiation was affected by the nature of thesurface exposed. The action of a blackened surface oftin being loo, that of a steel plate was 15, of clean tin 12,of tin scraped bright 10, when scraped with the edge of afine file in one direction 26, when scraped again acrossabout 13, a surface of lead clean 19 , covered with a greycrust 45, a thin coat of isinglass 80, resin 96 , writing paper98 , ice 85. Meat as well as light is so projected from asurface as to be equally dense in all directions, consequentlyfrom each point in a quantity which is as the sine of theangle of inclination. The radiationis not affected by thequality of the gas, in contact w T ith the surface, but it is nottransmitted by water. For the time of cooling of a hol-low tin ball 0 inches in diameter, filled with W'atcr, in

still air, take, in minutes, L.——L.

a h

making the

« + i

three first decimals integers, h and i being the tempera-tures on the centigrade scale, and « being 50 . And forthe same ball painted, make a ~ no, and take -fa of theresult: thus, from 100 ° C. to 50°, or 212 ° F. to 122 °,metal takes lai'.o, paper or paint 83'.2, to to° C. 01 50 °F. 002 ' and S44'.l respectively. For the effect of differentgases and different densities, in air the discharge from a

I I.

vitreous surface is *(sd 4 + 4<D°), from a metallic surface

it , 1

|(3 (l 4 4- ^d Tu ) ; in hydrogen gas J ,(l2d s + 4<M°), and

■((12 d b + I'd 5 ). Thus, if d ZZ 1 , theabductive power ofair is or .4280, the pulsatory energy of a vitreous surface.}, or . 57 14, of a metallic surface .fa, or ,0711. In hydrogengas the pulsatory power the same, the abductive power>,?, or 1 . 7143 . If d — ,i-, the abductive power is .18 (or air,.857 for hydrogen, the pulsatory power .48 or .00 in air,.51 or .0037 in hydrogen. If d the abductive

power is .1071 for air, .5655 for hydrogen, the pulsatorypower .433 or .054 for air, .475 or .0594 for hydrogen.It would be easy to make an experiment on the velocitywith which radiant heat, is conveyed to a distance, and

there is little ftoubt but that such an experiment wouldconfute Mr, Leslie's hypothesis of the transmission of heatby a pulsation of the air propagated with the velocity of

sound.