MODERN SCIENCE. 245 



the same sound in air as in water; the same is the case 

 with all gases ; any given sound depends on the number 

 of vibrations, and not on the nature of the sounding 

 body." The syren is so delicate that it has been found 

 by its means that the wings of a gnat flap 15,000 times 

 in a second. By combining the effects of the syren with 

 those of the chemical harmonicon, Tyndall has detected 

 very curious phenomena, such as that in which the 

 pulsations of the flame are the optical expressions of 

 the "beats" i.e., the alternate strengthening and weaken- 

 ing of two sounds acting on each other. Tyndall made 

 researches into the action of SOLAR and of ELECTRIC 

 LIGHT on attenuated vapours, and found that under such 

 conditions they are decomposed a discovery which put a 

 powerful agent of chemical decomposition into the hands 

 of chemists, and led the discoverer to important conclusions 

 regarding the origin of the blue colour of the sky and the 

 polarisation of daylight, these two phenomena being at 

 length accounted for. He also found the absorptive power 

 of several vapours, and discovered the class of phenomena 

 called by him DYNAMIC RADIATION AND ABSORPTION. 

 Tyndall, with Faraday, discovered the magnetism of gases, 

 and showed that very few substances, if any, are devoid of 

 magnetism. He made decisive researches upon the polar 

 character of the diamagnetic force. Being familiar with 

 every branch of physics, Tyndall has assisted science by 

 numerous investigations, but he is one of the chief instructors 

 of our age in sound, heat, light, and electricity. His works, 

 "Heat as a Mode of Motion," and "On Light/' are 

 universally known. 



18191868. Foucault applied electric light; proved the 

 velocity of LIGHT by striking experiments ; determined it in 

 a vacuum, in air, in water. By means of a rotating mirror, 

 which Wheatstone adopted in measuring the velocity of 

 electricity, he found light to travel at the rate of 185,157 

 MILES PER SECOND. He and Fizeau discovered the velocity 

 of light through water to be slower by one -fourth than through 

 air an additional proof of the wave -theory of light. He 

 also demonstrated by means of the PENDULUM the gradual 



