Theory of the Physical Forces, 451 



capable of giving of the phenomena of the radiometer, plane 

 and cup-shaped (Phil. Mag. for May and November 1876 and 

 April 1877), and also of the actions of the otheoscope and 

 telephone (Phil, Mag. for June 1878). I have adverted 

 to these productions for the purpose of making a remark rela- 

 tive to certain inferences which experimenters have drawn 

 from this novel class of phenomena, and which they designate 

 by the terms " the viscosity of residual gas," " the viscosity 

 of vacuum," " a fourth state of matter" in addition to the 

 solid, fluid, and gaseous. The explanations above cited appear 

 to me to point to the conclusiou that these terms, so far as 

 they are expressions of real phenomena, are referable to modes 

 of operation of the aetherial medium in which light and heat 

 are generated and transmitted, and are to be accounted for by 

 mathematical deductions from the hydrodynamical theory of 

 physical force. 



(6) The Theory of the Microphone I am about to propose 

 is a corollary from that of the Telephone given in the Philo- 

 sophical Magazine for June 1878. The following mode of 

 producing the phenomena of the microphone, which was con- 

 ducted in my presence, is convenient for describing the theo- 

 retical explanation. An oblong rectangular plate of charcoal 

 of moderate breadth and small thickness was approximately 

 balanced about its middle transverse section, and so placed 

 that the end of its heavier half rested slightly on another plate 

 of charcoal of greater breadth. An arrangement was made 

 by which the electric current pertaining to a telephone could 

 be passed through the two pieces of charcoal. Although my 

 hearing is not good, words transmitted from a distant room 

 were heard with sufficient loudness and distinctness by em- 

 ploying the telephone in the ordinary way, and applying a 

 receiving-cup to my ear. But as soon as the current was 

 made to traverse the pieces of charcoal, the loudness and di- 

 stinctness were marvellously increased, and I had no longer 

 need to apply the cup. This increment of sound may, I 

 think, be accounted for, on the principles of the hydrodyna- 

 mical theory of the telephone, in the following manner. The 

 current, in passing out of the narrower into the broader plate, 

 enters into a larger channel, and consequently, according to 

 the hydrodynamics of steady currents, its velocity is there 

 diminished and its density increased. The increment of pres- 

 sure thence resulting might suffice to raise the narrow plate ; 

 but if so, a very slight separation would so far intercept the 

 current as to cause a diminution of the lifting force, and the 

 plate would immediately be brought back by the force of gra- 

 vity. Thus a quick succession of partial interruptions would 



