PHYSICS. 253 



ty to light; but in both cases the former value of these con- 

 stants is gradually attained alter the current ceases to pass 

 through the substance. 



Mills has given the name electrostriction to a curious elec- 

 trical phenomenon, which may be produced as follows : The 

 bulb of an ordinary thermometer is first coated with silver 

 by a chemical method, and then with some other metal by 

 electrolytic deposition. The mercury will traverse some 

 portion of the scale, and finally take up a definite position 

 independent of temperature. Of the metals thus far experi- 

 mented with, copper, silver, iron, and nickel constrict the 

 bulb, while zinc and cadmium distend it. The author has 

 succeeded in determining: the electrostrictive effect in atmos- 

 pheres of pressure, and shows that, since the metal which 

 lias been deposited on the bulb may be removed by a chem- 

 ical solvent, it is possible to measure chemical action in 

 terms of atmospheres of pressure. 



4. Electric Spark and Light. 



Cazin has studied the spectrum of the electric spark taken 

 in compressed gas, both directly and by photography ; and 

 he concludes that the spark under these circumstances is 

 compound, containing incandescent gaseous particles pro- 

 ducing a spectrum of lines, and solid or liquid particles pro- 

 ducing a continuous spectrum. The first of these come from 

 the gaseous medium and from the electrodes ; the second 

 are torn from the electrodes or from the adjacent walls of 

 the tube. The solid or liquid particles are collected in the 

 central portions, the spark proper, while the aureole is form- 

 ed of gaseous particles. This aureole is to the total spark 

 what the bluish base of a candle flame is to the entire flame. 

 As the pressure increases, the solid or liquid particles be- 

 come more abundant, and their continuous spectrum pre- 

 dominates, finally extinguishing by its superior brightness 

 the linear spectrum of the gaseous portion. Hence he re- 

 gards it as incorrect to say that the gaseous lines widen and 

 unite to a continuous spectrum. 



Ayrton and Perry, in a letter to Nature, show that the 

 wire cage proposed by Maxwell as a protection against 

 lightning is not satisfactory, by quoting a case of lightning 

 in a coal-mine in India by which two miners were killed. 



