30 LECTURES TO SCIENCE TEACHERS. 



is indeed necessary to have some means of measuring the 

 conductivity of such substances. 



I will conclude by alluding to two extremely ingenious 

 pieces of apparatus which are exhibited here, which 

 illustrate a very important point. This is one founded on 

 a principle originally employed by M. Senarmont for 

 measuring the conductivity in crystals. He found that 

 the conductivity of crystals varied in different directions. 

 I will explain the application of this instrument, which is 

 on the same principle. There is a disc of glass upon which 

 the substance to be examined is laid, above it there is a 

 flat vessel through which water can be made to flow in 

 order to keep it at a constant temperature. There is a 

 platinum wire connected with the terminals of a battery 

 passing down and ending in a small bead of platinum. 

 The bead of platinum at the centre can be lowered down on 

 to the surface of the piece of rock. The rock is covered 

 with wax, and when the platinum is in contact with the 

 rock, and the wires are connected with the electric battery, 

 this has the effect of heating the platinum to an intense 

 heat, and consequently melting the wax on the surface of 

 the rock. Now the distance to which it will melt this 

 wax depends solely on the conductivity of the rock the 

 greater the conductivity the greater the distance to which 

 tha wax will be melted. But you will notice in all these 

 specimens that instead of having a circle of melted wax 

 round the plate which had been heated there is an ellipse 

 an oval curve showing distinctly that the conductivity 

 is greater in one direction than another. This instrument 

 is exactly on the same principle as Senarmont's, and the 

 results are exactly of the same nature as those arrived at 

 by him in the case of crystals. 



The conductivity of liquids has been very little inves- 

 tigated. There is great difficulty in it, owing to the 

 convection currents, and also owing to the amount of 

 radiation which passes through transparent substances. 

 Here is a perfectly new instrument invented by Dr. 

 Guthrie, which promises to give a very good idea of the 

 relative conductivity of different liquids. The principle of 

 it is extremely ingenious. There are two cones which are 

 separated by a very small distance. The upper one is con- 

 nected with these two tubes, through which steam can be 





