1851] WRITINGS OF JOSEPH HENRY. 305 



and magnetism, that we suppose this medium to be capable 

 of accumulation or condensation in certain portions of space, 

 and of being lessened in quantity or rarefied in other portions; 

 also, that in its return to its normal condition an actual 

 transfer of the medium takes place. It follows from these 

 assumptions that the fluid withdrawn from one portion of 

 space must leave an equivalent deficiency in another, or in 

 other words, that the amount of positive action must be 

 equal in all cases to that of the negative. Further, since 

 it appears from observation that the setherial medium can 

 only be condensed or accumulated in certain places by the 

 insulating powers of ordinary matter, no electrical phenomena 

 can be exhibited except in connection with such matter; 

 hence electrical action cannot be expected in the regions of 

 -celestial space. 



The most difficult phenomena for which to invent a plausi- 

 "ble mechanical explanation, connected with this subject, are 

 -those of the attraction of the two wires transmitting a current 

 of electricity, and the transverse action of a galvanic wire on 

 a magnetic needle. The theory of Ampere, though anadmira- 

 IdIc expression of a generalization of the phenomena of electro- 

 magnetism, is wanting in that strict analogy with known 

 mechanical actions which is desirable in a theory intended 

 to explain phenomena of this kind. 



In conclusion I would again revert to the importance, in 

 the adoption of mechanical hypotheses, of conditioning them 

 in strict accordance with the operations of matter under the 

 known laws of force and motion as exhibited in time and 

 space. 



20 



