290 Prof. Thomson on the Existence of a 



0= Si g V( a if=Si g t, e i,f; 

 0=SWw"j 0=(Wct') 2 



.}•'•• (256) 



results which may be compared with some of those obtained in 

 Section IV., for the two particular quine-sy stems, (A) and (B). 



Observatory of Trinity College, Dublin, 

 February 14, 1855. 



[To be continued.] 



XLIV. Observations on the " Magnetic Medium" and on the 

 Effects of Compression. By Professor W. Thomson. 



THE following letter was received a few days ago. It was 

 not written for publication, but the subject to which it 

 refers being of general interest at present, I ventured to suggest 

 to Professor Thomson the desirableness of having the letter 

 printed. Tbis he at once agreed to. With the exception of a 

 paragraph relating to matters of a purely private nature, the 

 letter appears as I received it. John Tyndall. 



March 24, 1855. 



2 College, Glasgow, 

 My dear Sir, March 12, 1855. 



Allow me to thank you for the abstract of your lecture on 

 magnetism, and the copy of your letter to Mr. Faraday, which 

 I have recently received from you, and have read with much 

 interest. I am still strongly disposed to believe in the magnetic 

 character of the medium occupying space, and I am not sure 

 but that your last argument in favour of the reverse bodily 

 polarity of diamagnetics may be turned to support the theory of 

 universally direct polarity. There is no doubt but that the 

 medium occupying interplanetary space, and the best approxi- 

 mations to vacuum which we can make, have perfectly decided 

 mechanical qualities, and among others, that of being able to 

 transmit mechanical energy in enormous quantities (a platinum 

 wire, for instance, kept incandescent by a galvanic current in the 

 receiver of an air-pump, emits to the glass and external bodies 

 the whole mechanical value of the energy of current spent in 

 overcoming its galvanic resistance). Some of these properties 

 differ but little from those of air or oxygen at an ordinary baro- 

 metric pressure. Why not, then, the magnetic property ? (of 

 which we know so little that we have no right to pronounce a 

 negative). Displace the interplanetary medium by oxygen, and 

 you have a slight increase of magnetic polarity in the. locality 

 with a drawing in of the lines of force. Displace it with a piece 

 of bismuth or a piece of wood, and a slight decrease of magnetic 

 polarity through the locality takes place, accompanied by a push- 

 ing out of the lines of force. A state of strain by compression 



