372

Thomson's theory of combustion.

and Pelletier observed long ago, that when water is poured uponlime not only heat but light is emitted. Light is emitted alsoabundantly when sulphuric acid is poured upon magnesia or uponlime, potash, or soda, freed from the water of crystallization,(page 134). In all these cases a semi-combustion takes place.The water and the acid being solidified give out caloric, whilethe lime or potash gives out light.

That lime during its burning combines with light, and thatlight is a component part of lime is demonstrated by the follow-ing experiment, for which we are indebted to Scheele.

It has been mentioned already (page 131), that fluor spar(fluate of lime) has the property of phosphorescing strongly whenheated, but the experiment does not succeed twice with thesame specimen. After it has been once heated sufficiently, nosubsequent heat will cause it to phosphorate. Now phosphores-cence is merely the emission of light, light of course is a compo-nent part of fluor spar, and heat has the property of separatingit. But the phosphorescing quality of the spar may be again re-covered to it, or which is the same thing, the light which thespar had lost may be restored by the following process:

Decompose the fluate of lime by sulphuric acid, and preservethe fluoric acid separate, (page 344). Boil the sulphate of limethus formed with a sufficient quantity of carbonate of soda; adouble decomposition takes place; sulphate of soda remains insolution, and carbonate of lime precipitates. Ignite this preci-pitate in a crucible till it is reduced to lime, and combine it withthe fluoric acid to which it was formerly united. The fluor sparthus regenerated phosphoresces as at first. Hence the lime dur-ing its ignition must have combined with light.

That potash contains light, may be proved in the same manneras the existence of that body in lime. Now as potash is deprivedof its carbonic acid by lime, the Doctor supposes that the processmust be a double decomposition; namely, that the base of thelime combines with carbonic acid, while its light combines withthe potash.