382 EXPERIMENTAL INVESTIGATIONS. 



Now, when the points are separated to the maximum for 

 the experiment which I have detailed above, two classes of 

 sparks will be perceived at different times in the air the one 

 a thin blue spark, giving to the ear a single sharp sound, and 

 the other a burred yellow spark, with a sound not so clear and 

 metallic, but attended with a slight whizz : this contrast of 

 sound may be roughly compared to that presented by a whole 

 or cracked piece of porcelain when struck ; it requires attention 

 and habit to distinguish these two classes ; but when the 

 power is attained, it will be seen that with the first or single 

 discharge there are no striae in the receiver ; with the second, 

 which I believe to be a double discharge, striae are visible. 



By careful adjustment I have been able to obtain sparks 

 of the former class only, and then the experiment may be 

 continued indefinitely with no striae visible ; but on the slightest 

 alteration in the distance the striae appear. It is, however, 

 more frequently the case that, notwithstanding all the care 

 used, sparks of both characters will pass in irregular succes- 

 sion ; and then the discharges will be without striae in some 

 cases, and with them in others. 



It is necessary to mention the above distinctions, as other- 

 wise any person repeating the experiments might be deceived, 

 and not succeeding in obtaining discharges without striae, 

 though the discharges were apparently single, would ima- 

 gine the results I have described to be illusions arising from 

 an error of vision. 



I repeated the above experiments in one of Mr. Gassiot's 

 vacuum tubes, which showed the striae beautifully under ordi- 

 nary circumstances ; but when the division in the circuit was 

 carefully made and carried to its fullest extent, the discharges 

 passed without any striae, the tube being filled at each dis- 

 charge with a uniform glow. 



From the above experiments I am satisfied that in atten- 

 uated media wherein the striae are well seen when the ordinary 

 interrupted currents are used, discharges may be made to pass 

 which exhibit no striae, and under such circumstances that 

 there is every reason to believe single discharges only pass. 

 Join to this the fact that the appearances of the striaa vary 

 with any variation of the contact-breaker, that the striae are 



