Item 125 - Letter from William Thomson to James David Forbes

8 pp.

Glasgow - WT [later Lord Kelvin] attended Michael Faraday's lecture on Gravitation, and spoke to both him and John Tyndall: 'I made a slight attack on Tyndall by asking him to explain to me the distinction between a viscous solid and a plastic solid. He said that before the end of a year it would be very clear. Which ever word is the most appropriate is the best expression of your theory as I have always understood it. As to the clear and porous alternate layers proving the veined structure, I do not know whether you lay much stress on the explanation Tyndall quotes as yours. It may be true what Tyndall says - that it is occasioned by pressure but that is no explanation'. Many writers have assumed that pressure is the cause of the clearage in slate mountains: 'It is a real thing proved if Tyndall or any one else can prove the clearage surfaces to be perpendicular to the lines of maximum compression'. In diamagnetics WT holds that Weber [Wilhelm Weber] and Tyndall have illustrated by experiment conclusions deducible (and which I deduced in 1846) from Faraday's forces experienced by bismuth; that they have established no new conclusion'. Faraday does not seem to perceive the relation with Weber's phenomena and even doubts Weber's results; 'Tyndall's repetition of Weber's experiment (described in the Phil. Trans.) confirmed the results and removed the possibility of such doubts as Faraday had temporarily raised. Not one of these experiments touches the ultimate nature of the magnetic effect experienced by the substance of a piece of bismuth, since the resultant external action is necessaily the same whether air in the surrounding medium is unpolarised and bismuth severly, or the surrounding medium and the substance of the bismuth both polarised directly (like a 'paramagnetic') but the surrounding medium more so than the bismuth'. Many of Tyndall's experiments simply prove things that did not require proving: 'In reality no testing experiment has ever been made to distinguish between two hypothesis: and I agree (I believe) with Faraday in thinking the second the more probable of the two true (I had a good deal of this in a letter to Tyndall which he published in the Phil. Mag. April 1855)'. WT has been occupied chiefly with electrometers and electroscopes in the apparatus room.