98 Prof. Potter on Mr. Alexander's formula 



d. If the silk is undried and there is dust flying about, the 

 same experiment may be made without any apparent charge to 

 the ball, because the convective charge may neutralize the induc- 

 tive ; but the escape of the negative up the silk line to the elec- 

 troscope reveals the fact that convection and induction are both 

 at work. 



49. Cylinders and plates of sulphur, shell lac, and heated 

 glass coated with melted shell lac were suspended by dried silk 

 lines, and subjected to the inductive influence of an excited rod 

 of shell lac 18 inches long and 1J inch in diameter. They 

 turned and were attracted like insulated conducting bodies of 

 the same shape, but with less force. 



The non-conducting quality of these substances was tested 

 previously to experiment by placing them on the top plate of the 

 electroscope while charged to its full extent, then touching them 

 with free conductor; the gold-leaves, watched closely for some 

 minutes, showed not the slightest change. 



50. A ribbon of the finest sort of shell lac was drawn out 

 about 3 feet long, ^ of an inch broad, and ^Q-^th. °f an mcn thick. 

 While hanging freely by one end, an excited cylinder of shell lac 

 was brought within a few inches, and attraction was strongly 

 manifested : a thread of dried silk is similarly attracted. A 

 piece of the ribbon was then laid upon plate of electroscope 

 when it was fully charged. A needle or free conductor was 

 then brought down to touch the top surface of the ribbon 

 within about T ^o tn °f an i ncn °f tne plate. The leaves, watched 

 closely for a minute or two, showed no loss. 



50 J. AB.D. charged to 90° x 4 was placed with its rounded 

 edge close to the end of a small stick of lac that had just been 

 melted by flame of taper. A small cone immediately rose from 

 the melted surface. (See full-size figure 24|.) 

 [To be continued.] 



XV. An examination of the applicability of Mr. Alexander's 

 formula for the elastic force of Steam, to the elastic force of the 

 vapours of the Liquids as found by the experiments of M. Reg- 

 nault. By Professor Potter, A.M.* 



TO those who are investigating the development of the me- 

 chanical force which arises from the action of the subtile 

 agents causing the phenomena of heat, electricity, magnetism, 



veyance by floating particles, I am inclined to believe that the silk lines 

 employed conducted slowly, as no mention is made of their having been 

 specially dried before making the experiment ; and unless they were so 

 they could not prevent an inductive charge. 

 * Communicated by the Author. 



