- Mr. Crookes has applied his remarkable discovery that radiated
light exercises a repelling force in a vacnum to weigh the light of a candle. He uses the repelling power so as to twist a certain glass thread round and round, and the number of turns required to untwist it again is registered on a scale, and supplies a measure of the twisting power exerted. A little iron weight of one-hundredth of a grain which is within the tube is lifted by a magnet on to the little cross-bar, and the torsion caused by its weight is observed in the manner explained. Then the light of the candle was tried in the same way, and it was found that while the iron weight of a hundredth of a grain caused a torsion of 10.021 degrees, the candle caused a torsion of only 1.628 degrees, or a torsion between a sixth and a seventh of that due to the iron bar. In other words, the impulse due to the light of the candle was weighed. Mr. Crookes calculates that the force exerted by- the sun's light is equal to 32 grains on the square foot, or 57 tons on the square mile, or three thousand millions of tons on the whole earth, a repellent force which, but for the far more powerful attractive force of gravitation, would drive the earth off into space. As not a little of the invigorating effect of fine weather is said to be due to the increased pressure of the atmosphere, so it is possible that the stimulus certainly given by light to the nervous system may be partly due to this minute pressure.
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