136 Mr. S. Bid well on the Effect of Heat 



so than when dilated with C0 2 . The difference between the 

 results for HI may be explained in the same way ; and the 

 case of ammonium chloride is practically the same as HC1, as it 

 dissociates into NH 3 and HC1, and the NH 3 does not conduct. 



I may say that it was the fear of the true conductivity of 

 the hot gases being masked by secondary effects of this kind, 

 combined with the high conductivity of the flame itself, which 

 made me reject the use of flames in my experiments. In a 

 flame the chemical conditions are so complex that it is almost 

 impossible to obtain results whose interpretation is free from 

 ambiguity. 



Some other remarks of Dr. Arrhenius are due to a mis- 

 interpretation of my meaning. I never stated that if we had 

 the same number of molecules per unit volume of HI and 

 I, the HI would conduct better than the I. On the contrary, 

 I regard the conductivity of both HC1 and HI as due to some 

 of the chlorine and iodine remaining dissociated after the 

 molecules of the gas have been split up. Dr. Arrhenius also 

 explains the small conductivity of a dissociated gas, such as 

 iodine, when compared with the conductivity of an electro- 

 lyte, by asking, Who can tell how many of the atoms of the 

 dissociated gas are uncharged ? but if the atoms of the gas 

 can be without charge why cannot those of a salt in solution ? 

 and if we admit the existence of uncharged, that is electrically 

 inactive atoms, why should there be any connexion between 

 electric conductivity and osmotic pressure ? 

 I remain, Gentlemen, 



Yours very truly, 



J. J. Thomson. 



XVII. A Lecture Experiment illustrating the Effect of Heat 

 upon the Magnetic Susceptibility of JSickel. By Shelfokd 

 Bidwell, M.A., F.R.S* 



IT is well known that iron when made red-hot loses its sus- 

 ceptibility and practically becomes a non-magnetic metal. 

 Nickel becomes non-magnetizable at a much lower tempera- 

 ture, perhaps at about 300°. The following is a description 

 of a simple piece of apparatus by means of which this effect 

 may be shown as a lecture experiment. 



A copper disk, to which a thin projecting tongue of nickel 

 is soldered, hangs like the bob of a pendulum from a double 

 thread. This bob is held on one side by a horizontally-fixed 

 bar-magnet, which attracts and holds fast the nickel tongue 

 when brought into contact with it. A spirit-lamp is placed 

 * Communicated by the Physical Society : read April 13, 1889. 



