263 



the room, by the evaporation of sulphuric ether, their contact 

 continually produced a western deviation, which, by rubbing, 

 was instantaneously changed into a contrary or eastern one, 

 of apparently the same amount as before, and this likewise 

 remained stationary as long as the friction continued ; but by 

 the interruption of it, the western deviation was immediately 

 restored. This simple sketch of the phenomena of changes 

 of intensity, or even of sign, which friction at the point of con- 

 tact gives to the deviation of a multiplicator's needle, will 

 already suffice to exhibit it as a mere consequence of the heat 

 produced by the action of rubbing." 



Mr. Donovan expressed his opinion that Professor Erman 

 and the authorities to whom he alludes, have been deceived 

 in their conclusions by their not having been in possession of 

 a sufficient number of facts. After an examination of all the 

 phenomena which had been discovered by himself and others 

 in this department, Mr. Donovan stated his opinion that they 

 are all dependent on the following eighteen general principles 

 or laws, most of which have now, for the first time, been 

 developed : — 



I. The agent which causes the deflection of the needle of 

 the galvanometer may be brought into action either by the 

 attrition or thermo-contact of certain metals, metallic ores, or 

 certain forms of carbon. 



II. When two different metals, and sometimes separated 

 masses of the same metal, are rubbed against each other, de- 

 flection will result, the degree and direction of which will 

 vary with the metals employed, with the force and rapidity of 

 attrition, and in some cases with the temperature. This de- 

 flection will take place in air, or under the surface of mercury, 

 or of aqueous, oily, ethereal, or alcoholic liquids. 



III. When two difi'erent metals, and sometimes two sepa- 

 rate masses of the same metal, or even when two difi'erent 

 parts of the same mass of metal, are brought in contact at 

 unequal temperatures, deflection will take place, the degree 



