I873-] 



Heat. 141 



HEAT. 



Prof. Volpicelli, in " Poggendorff s Annalen," says — It has' been asserted 

 that a lowering of temperature is produced when air, which has been com- 

 pressed in a vessel, is allowed to stream out against the surface of a thermo- 

 pile. To test this assertion I compressed air in a cylindrical vessel to four 

 atmospheres, and, after the heat of the compression had disappeared, I allowed 

 the air to stream against a thermopile, which was connected with a reflecting 

 galvanometer. Three different results appeared. If the commencement of 

 the air-stream was pretty near the surface of the pile there was elevation of 

 temperature, if it was somewhat distant from the surface the temperature fell, 

 and at a point intermediate there was no change of temperature — the image 

 reflected from the needle was unmoved. These results may also be obtained 

 if air is blown, with an ordinary bellows, against the surface of the pile ; only 

 in this case the rise or fall of temperature is less marked, owing to the smaller 

 compression of air. I also obtained the results, though in still less measure, 

 with a centrifugal ventilator. These three results are quite in accordance with 

 the new thermodynamic theory. In the experiments the causes of variation 

 of temperature are of three kinds : — One consists of the destruction of the 

 vis viva of the air, or external work ; a second consists of internal work, done 

 by the air molecules which become condensed in the pores of the metal of the 

 pile ; and the third of external work, done by the molecules as they expand in 

 their course. The two first cause an elevation, the third a lowering of tem- 

 perature. It is thus seen how one or other of the three above-described results 

 is produced, according as the effects of the two first causes are greater or smaller 

 than the opposing effect of the third cause, or equal to it. Remove the source 

 of the air-stream and you have, first, a zero point of increase of temperature, 

 then a decrease of temperature. Remove still further, and you come to a 

 zero point of increase of temperature. From this is to be inferred that 

 between these two distances (corresponding to the two zeros) there is a max- 

 imum of decrease of temperature which the galvanometer indicates. If it 

 were possible to drive the air against the pile without compressing it, and, 

 therefore, without expansion taking place, the two first causes only would 

 operate and there would be heat produced. But these conditions are unattain- 

 able. In order to show to a large audience the transformation of destroyed 

 vis viva into heat, I suspended a ball of phosphorus near a wall, and standing 

 about 10 metres off, blew the ball with a pair of bellows against the wall, the 

 ball was set on fire when it struck, not in its passage through the air. In 

 another experiment I let a solid body fall on the surface of the thermopile, the 

 latter being connected with a reflecting galvanometer. The reflected image 

 was then seen to move several degrees over the scale, indicating elevation of 

 temperature. This experiment is quicker and more simple than that sometimes 

 performed in which a body is allowed to fall several times from a certain height 

 on a hard substance, and then applied to the pile. 



ELECTRICITY. 



Dr. G. Robinson has recently patented a new method of sawing timber. It 

 consists in applying a platinum wire, heated to redness or whiteness by an 

 electric current, to the trees or wood which are to be severed much in the same 

 manner as it has hitherto been employed in removing tumours from the human 

 subject. By fitting the wire with handles so as to be able to guide it in any 

 direction the most intricate fretwork can be cut. 



M. J. Jamin has contributed to the French Academy of Sciences a paper, in 

 which he shows that magnetism may be condensed in a manner similar to 

 electricity. Having for some special purposes had a large horseshoe magnet 

 made, consisting often lamina? of perfectly homogeneous steel, each weighing 

 ten kilogrammes, he suspended it to a hook attached to a strong beam, and 

 having wound copper wire round each of the legs, which were turned down- 

 wards, he put the latter into communication with a battery of fifty Bunsen's 



