804 Royal Institution, 



know what takes place ; the last ball will fly away, the intervening 

 balls having served for the transmission of the shock from one end 

 of the series to the other. Or we might refer to the conduction of 

 heat. If, for example, it be required to transmit heat from the lire 

 to a point at some distance from the fire, this may be effected by 

 means of a conducting body — by the poker for instance : thrusting 

 one end of the poker into the fire it becomes heated, the heat makes 

 its way through the mass, and finally manifests itself at the other 

 end. Let us endeavour to get a distinct idea of what we here call 

 heat ; let us first picture it to ourselves as an agent apart from the 

 mass of the conductor, making its way among the particles of the 

 latter, jumping from atom to atom, and thus converting them into a 

 kind of stepping-stones to assist its progress. It is a probable con- 

 clusion, even had we not a single experiment to support it, that the 

 mode of transmission must, in some measure, depend upon the man- 

 ner in which those little molecular stepping-stones are arranged. 

 But we need not confine ourselves to the material theory of heat. 

 Assuming the hypothesis which is now gaining ground, that heat, 

 instead of being an agent apart from ordinary matter, consists in a 

 motion of the material particles ; the conclusion is equally probable 

 that the transmission of the motion must be influenced by the man- 

 ner in which the particles are arranged. Does experimental science 

 furnish us with any corroboration of this inference ? It does. More 

 than twenty years ago MM. De la Rive and De CandoUe proved that 

 heat is transmitted through wood with a velocity almost twice as 

 great along the fibre as across it. This result has been recently 

 expanded, and it has been proved that this substance possesses three 

 axes of calorific conduction ; the first and greatest axis being parallel 

 to the fibre ; the second axis perpendicular to the fibre and to the 

 ligneous layers ; while the third axis, which marks the direction in 

 which the greatest resistance is oflfered to the passage of the heat, is 

 perpendicular to the fibre and parallel to the layers. 



But it is the modification of the magnetic force by the peculiari- 

 ties of aggregation, which forms the subject of the evening's 

 lecture. What has been stated regarding heat applies with equal 

 force to magnetism. ITie observed magnetic phsenomena are of a 

 composite character. The action of a magnetic mass is the re- 

 sultant action of its molecules, and will be influenced by the manner 

 in which they are aggregated. The fundamental phaenomena of mag- 

 netism are too well known to render it necessary to dwell upon 

 them for an instant. A small bar of iron was suspended in the 

 magnetic field ; it set its length parallel to the line joining the poles. 

 Should we be justified from this experiment in concluding that a mag- 

 netic mass will always set its longest dimension axial } No. A 

 second magnetic bar, equal in size to the former, was suspended 

 between the poles ; it set its length at right angles to the line 

 joining the poles. Whence this deportment } We find the reason 

 of it in the mechanical structure of the bar : it is composed of mag- 

 netic plates, transverse to its length ; these plates set from pole to 

 pole, and hence the length of the bar equatorisJ. But let us proceed 



