178 Dr. J. S. Lombard. On the Propagation of 



ductors of this size and character being more manageable in packing 

 the pile in paraffine in the manner to be described farther on.* 

 Second. — The writer's rheostat and keys.f 

 Third. — Sir William Thomson's galvanometer and scale. 

 The greatest precaution must be taken to guard against the 

 development, in any part of the apparatus, of accidental currents due 

 to external thermal influences. For this reason, not only should every 

 exposed junction of dissimilar — or even similar — metals be thoroughly 

 protected with cotton-wool, but also the whole rheostat and the keys 

 should be covered over with several layers of flannel, the plugs and 

 keys being manipulated through a single thickness of the cloth, the 

 other layers being momentarily raised for this purpose, and only at the 

 very point concerned. Moreover, besides covering thickly with wool 

 the binding screws of the Thomson galvanometer, the whole brass 

 back of the instrument should be covered with flannel extending over 

 the top and sides of the box containing the coil, and leaving only the 

 glass front exposed. 



Methods of Experimenting. 

 In earlier experiments (1867-68), in the case of bad conductors 

 generally, provided the substances were of sufficient density, a form 

 of apparatus similar to that used by Professor Tyndall in like investi- 

 gations was employed ;J but in later, including the present, experi- 

 ments, the methods adopted were different, and in the present instance 

 were of two kinds, both, however, the same in principle, and differing 

 only in detail. 



The fundamental principle of both methods was the determination 

 by means of a thermo-pile applied to one surface of the substance 

 under examination, — say, for example, a piece of bone, — of the 

 rapidity and extent of the change of temperature induced by conduc- 

 tion in this surface by the contact of the opposite surface with a mass 

 of water of a temperature differing in a slight but definite degree from 

 that of the air in the immediate neighbourhood. At the outset, the 

 whole of the piece of bone and the pile, if properly protected, will be 

 at the temperature of the surrounding air ; and when contact of one 

 surface of the bone with the water takes place, this surface, assuming 

 the temperature of the water gives rise to a thermal movement across 

 the bone proportional to the difference of temperature between its two 

 surfaces, and as these two surfaces are now respectively at the tem- 

 peratures of the air and of the water, the movement is proportional to 

 the difference between the latter two temperatures. 



* All possibility of currents caused by vibration of the conducting wires must be 

 guarded against, hence larger wires than those specified, unless flexible like strands, 

 are unsafe. 



i Op. ext., p. 22. 



% " Heat considered as a Mode of Motion," American ed., p. 233. 



