Mechanical Science. 3fi9 



Iron, submitted to successive blows, also becomes luminous. 

 Are not the same electric phenomena of pressure produced 

 here, as when two masses of ice strike together.' 



6. Developement of Electricity br; two pieces of the same metal. — 

 Among the applications of the electro-magnetic multiplier, is 

 the following : — If two pieces of the same metal are plunged, at 

 different moments, into an acid capable of acting on them, that 

 which was first introduced will act as the most positive metal 

 to the other. The experiment may be made very well with 

 zinc and diluted muriatic, or sulphuric acid. — Avogadro, Art' 

 nalesde Chim. 



7. Variation of Thermometers. — In the last volume of this 

 Journal, p. 441, notice was taken of an observation made by 

 M. Flaugergues on the instability of the freezing point of ice, as 

 laid down on thermometers. The effect was not observed in alco- 

 hol thermometers or in mercurial thermometers open at the top, 

 and was attributed to the gradual yielding of the glass bulb to 

 the external atmospheric pressure, which, diminishing its bulk, 

 raised the surface of the mercury in the tube, and rendered the 

 ecale incorrect. 



M. Bellani has entered into the investigation of an analogous 

 error in thermometers, and published the result of his researches 

 in the Giornate di Fisica, v. 268. He finds that a mercurial ther- 

 mometer, being made in the usual manner, and the freezing 

 point of water marked on it from experiment, if it be laid 

 aside awhile, and again plunged in melting ice, the mercury will 

 stand higher than before ; and that if it be put aside again, and 

 then again tried, the mercury will be higher still, until, at the end 

 of a certain time, a year or so, the effect of elevation will cease. 



It was found from numerous experiments, that the result was 

 not influenced by the various qualities of the glass used in the 

 instrument; by the more or less perfect exclusion of air from the 

 bulb or tube; by the constant horizontal, perpendicular, or in- 

 verted position of the instrument ; by the open or closed ex- 

 tremity; by the longer or shorter time of remaining in the ice ; 

 or by the compression of the surrounding ice. Neither was it 

 found to be peculiar to mercurial thermometers, but was ex- 

 hibited by alcohol thermometers, though in a less degree. 



M. Bellani at last ascertained, that the effect was due to a 

 gradual and slow contraction of the glass after having been 

 highly heated, which contraction, as long as it continued, di- 

 minished the bulk of the instrument, and consequently forced 

 the fluid into the tube. This effect he illustrates in the follow- 

 ing manner: — Take a Florence flask, or any similar thin glass 

 vessel, such as a matrass with a long narrow neck, shortly after 

 it has come from the glass furnace, it not having been annealed 

 2 B2 



