A system of chemistry, in four volumes (Volume 1).

  • Thomas Thomson
Date:
1818
Catalogue details

Licence: Public Domain Mark

Credit: A system of chemistry, in four volumes (Volume 1). Source: Wellcome Collection.

Provider: This material has been provided by the National Library of Medicine (U.S.), through the Medical Heritage Library. The original may be consulted at the National Library of Medicine (U.S.)

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    A red hot body continues to shine for some time after it has been taken from the fire and put into a dark place. The constant acces- sion, then, either of light or heat, is not necessary for the shining of bodies: but if a red hot body be blown upon by a strong cur- rent of air, it immediately ceases to shine.* Consequently the moment the temperature of a body is diminished by a certain num- ber of degrees, it ceases to be luminous. Whenever a body reaches the proper temperature, it becomes luminous, independent of any contact of air; for a piece of iron wire becomes red hot while immersed in melted lead.f^: To this general law there is one remarkable exception. It does not appear that the gases become luminous even at a much higher temperature. The following ingenious experiment of Mr. T. Wedgewood seems to set the truth of this exception in a very clear point of view. He took an earthen ware tube, bent so in the mid- dle that it could be sunk, and make several turns in a large cruci- ble, which was filled with sand. To one end of this tube was fixed a pair of bellows; at the other end was a globular vessel, in which was a passage, furnished with a valve to allow air to pass out, but none to enter. There was another opening in this globular vessel filled with glass, that one might see what was going on within. The crucible was put into a fire; and after the sand had become red hot, air was blown through the earthen tube by means of the bellows. This air, after passing through the red hot sand, came into the globular vessel. It did not shine; but when a piece of gold wire was hung at that part of the vessel where the earthen ware tube entered, it became faintly luminous: a proof that though the air was not luminous, it had been hot enough to raise other bo- dies to the shining temperature. 20. The last of the sources of light is percussion. It is well known, that when flint and steel are smartly struck against each other, a spark always makes its appearance, which is capable of set- ting fire to tinder or to gunpowder. The spark in this case, as was long ago ascertained by Dr. Hooke, is a small particle of the iron, which is driven off, and catches fire during its passage through the air. This, therefore, and all similar cases, belong to the class of combustion. But light often makes its appearance when two bo- dies are struck against each other, when we are certain that no such thing as combustion can happen, because both the bodies are in- combustible. Thus-, for instance, sparks are emitted, when two quartz stones are struck smartly against each oth'er, and light is emitted when they are rubbed against each other. The experiment succeeds equally well under water. Many other hard stones also emit sparks in the same circumstances. * T. Wedgewood, Phil. Trans. 1792. t Id-Ibid- + [All species of fuel used, is vegetable matter: during the combustion, light previously combined with the vegetable during its living state, is set free: may not the light thus set free from the fuel, be absorbed by the heated metal, and occasion its liyuinous appear- ance?—C]