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42 sulphide, their effect must increase slowly, and that when they are suppressed, their effect can only disappear slowly.

Pebbles picked up at about four o'clock p.m., in a yard where they had been exposed to the sun, spontaneously emitted "N" rays; bringing them near a small mass of phosphorescent sulphide was sufficient to increase its luminosity. Fragments of calcareous stone, brick, etc., picked up in the same yard, produced analogous actions.

The activity of all these bodies still persisted after four days, without any sensible diminution. It is, however, necessary for the manifestation of such actions that the surface of these bodies should be quite dry; for we know that the thinnest layer of moisture is sufficient to arrest "N" rays. Vegetable earth was found to be inactive, doubtless on account of its moisture; pebbles taken from several centimetres underneath the surface of the soil were inactive, even after being dried.