physics. 597 



be 6991 calories referred to Br 2 , molecular weight, 160. (Ann. Chim. 

 Phys., November, 1883, V, xxx, 382, 400, 410.) 



Frankland has contrived an instrument for registering the relative 

 thermal intensity of the sun. It consists of two bulbs at the ends of a 

 tube bent twice at right angles, resembling the differential thermometer 

 of Leslie. These bulbs have the same diameter, and one of them is 

 blackened and surrounded by a glass envelope, which is exhausted. 

 The other bulb is placed beneath a zinc roof painted with zinc-white on 

 both faces. The apparatus contains air, and the tube is partially filled 

 with mercury. The blackened bulb receives the solar radiation, the 

 other preserves the temperature of the surrounding air. The reading 

 of the two mercury columns on a suitable scale gives the difference of 

 the temperatures. (Proc. Roy. Soc, xxxni, 331; J. Phys., February, 

 1883, II, ii, 93.) 



LIGHT. 



1. Production and Velocity. 



Lodge has given an interesting lecture at the London Institution on 

 the ether and its functions. Light vibrations, he says, can be trans- 

 mitted only by a body possessing rigidity; and rigidity is active resist- 

 ance to shearing stress, to alteration of form. Elasticity of figure is 

 possessed by solids alone; the elasticity of fluids is volume elasticity 

 only. Hence, fluids can transmit longitudinal vibrations only, while 

 solids alone can transmit transverse vibrations like those of light. 

 Water and air, therefore, cannot transmit light vibrations; it is the 

 ether in them which conveys the motion. At 4,000 miles above the 

 earth's surface the density of the air is represented by a number with 

 127 ciphers before it and after the decimal point. But according to 

 Sir William Thomson's calculation the density of the ether is repre- 

 sented by a number with only 17 ciphers between it and the decimal 

 point. The rigidity being the product of the square of the velocity by 

 the density, is therefore 900, while that of steel is 8 x 10 11 . Glass itself 

 can transmit vibrations with a velocity of only half a million centimeters 

 per second, but the ether in the glass transmits them 40,000 times as 

 quick, or 20,000,000,000 centimeters per second. Outside the glass they 

 are transmitted 30,000,000,000 centimeters per second. Fresnel assumed 

 the ether to be really denser within ordinary matter, being condensed 

 around the molecules, while the rigidity is unchanged. Hence it fol- 

 lows that in water, for example, seven-sixteenths of the ether within it 

 is bound to the molecules and moves with it, while the remaining nine- 

 sixteenths is free aud blows freely through the mass. The electric rela- 

 tions of the ether are discussed and the suggestion made that, since a 

 given electromotive force produces a greater electric displacement in 

 some kinds of matter than in others, i. e., that the electricity is denser 

 in some kinds of matter, the ether is sheared by electromotive forces 

 into positive and negative electrification. The density of electricity in 



