A text-book of inorganic chemistry / by G.S. Newth.

  • Newth, G. S. (George Samuel)
Date:
1895
Catalogue details

Licence: Public Domain Mark

Credit: A text-book of inorganic chemistry / by G.S. Newth. Source: Wellcome Collection.

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    the power to emit a feeble light when seen in the dark. A stick of phosphorus also emits a very similar light when seen in the dark. The glowing of these materials under these circumstances might readily be regarded as the result of the same kind of change in both cases, but in reality the luminosity of the phosphorus is due to a chemical change taking place upon the surface of that sub- stance, while the emission of light from the luminous paint is a purely physical phenomenon. The two sciences, chemistry and physics, are so closely related and interdependent upon each other, that no sharp distinction or line of separation between them is possible. Every che7nical change that takes place is attended by some physical change, and it often happens that this accompanying physical change forms the only indication of the chemical change that has taken place. In certain important points, however, a chemical change is very different from one that is purely physical: in the latter case no material alteration in the essential nature of the substance takes place. This will be seen in the examples quoted. The steel needle remains unaltered in its essence, although by magnetisation it has acquired a new property, a property which it again loses, and which can be again and again imparted to it. The match, on the other hand, when ignited has undergone a material and per- manent change : the combustible substance is now no longer combustible, neither will it ever return to its original state. The solid water, in being transformed to liquid water, has not under- gone any vital change : in essence it is the same substance merely endowed with a new property of liquidity, a property which it loses again when cooled, and which can be again and again imparted to it : on the other hand, the coagulated albumen has undergone a complete and lasting change, and never returns to its original condition. In the same way, the luminous paint gradually ceases to emit light, and returns to its original state; it may be exposed to the influence of light, when it once more acquires the property of phosphorescence, and this change may be brought about indefi- nitely. without altering the intrinsic nature of the substance. The glowing phosphorus, on the other hand, is gradually changed into a white substance, which escapes from it as a smoke or fume ; in the act of glowing the phosphorus is undergoing a process of slow burning, and if allowed to remain will continue glowing and burn- ing until the whole of it has disappeared in the form of smoke.