XXI 



field was due, not to a force previously unknown, hut to a modifica- 

 tion of the known forces of magnetism and diamagnetism by crystal- 

 line aggregation. They found, for instance, that whilst Iceland 

 spar, which had been adduced by Pliicker and experimented on by 

 Faraday, was, according to the law of Pliicker, axially repelled by a 

 magnet, it was only necessary to substitute, in whole or in part, 

 ferrous carbonate for calcic carbonate, thus changing the magnetic 

 but not the optical character of the crystal, to cause the axis to be 

 attracted. They proved that the deportment of magnetic crystals is 

 exactly antithetical to that of diamagnetic crystals isomorphous with 

 the magnetic crystals, and showed this to be a general law. In all 

 cases, the line which in a diamagnetic crystal set equatorially, always 

 set itself in an isomorphous crystal axially. It was, moreover r shown 

 that by mechanical compression other bodies were also made to imi- 

 tate Iceland spar. The results of these experiments were published 

 in the ' Philosophical Magazine ' and in ' Poggendorff's Annalen,' and 

 Tyndall subsequently, and apart from Knoblauch, continued these 

 investigations in the laboratory of Magnus. The results and the 

 conclusions drawn from them which form the first section of Tyndall's 

 experimental researches, have never been called in question. 



The great work of Tyndall's life, however, was not performed in 

 the domain of magnetism, but in that of heat. Already, in the year 

 1852, he was experimenting on the transmission of heat through 

 organic structures, and in OctobeV of that year he sent his first paper 

 to the Royal Society, entitled " On Molecular Influences. Part I. 

 Transmission of Heat through Organic Structures." As an illustra- 

 tion of the leisurely way in which such papers were treated in those 

 days, this was not read until the 6th of January in the following 

 year, and did not appear in the l Transactions ' until tbe year 1854, a 

 year after it was read. The paper deals with the transmission of 

 heat through wood, and the author expresses the laws of molecular 

 action which he deduces from his experiments as follows : 1. At all 

 the points not situate in the centre of the tree, wood possesses three 

 unequal axes of calorific conduction, which are at right angles to 

 each other. The first and principal axis is parallel to the fibre of the 

 wood. The second and intermediate axis is perpendicular to the fibre 

 and to the ligneous layers ; while the third and least axis is perpen- 

 dicular to the fibre and parallel to the layers. 2. Wood possesses 

 three axes of cohesion which coincide with the axes of calorific con- 

 ductionthe greatest with the greatest, and the least with the least. 

 3. Wood possesses three axes of fluid permeability which coincide 

 with those of calorific conduction the greatest with the greatest, and 

 the least with the least. 



These researches on the transmission of heat through organic 

 structures were not afterwards continued, as Tyndall's attention was 



