of Edinburgh, Session 1878—79* 51 
vious experiment ; the point was conical and of brass. From 1 to 
5 mm. the discharge was in the form of a white spark; for higher 
distances nothing was visible excepting a glow at the point. The 
series was continued up to 200 mm., as there was no difficulty due 
to escape of the electricity into the air. 
Discharge through a Solid Dielectric . — We obtained, by favour 
of Mr Calderwood, of Addiewell Chemical Works, a quantity of a 
pure solid paraffin of low melting point. The plate electrodes were 
separated to a distance of J inch inside a glass vessel, the liquefied 
paraffin poured in so as to cover the plates completely, and then 
allowed to solidify for twenty-four hours. When the plate elec- 
trodes were charged the first spark which passed was large and 
illuminated the whole of the paraffin; but the succeeding discharges 
were much smaller, and of equal amount. The first spark produced 
a deflection 3*6 times as great as the succeeding sparks. When the 
plate of paraffin was examined afterwards, it was found to be per- 
forated in a zigzag manner, the hole being surrounded by char. We 
found that— 
Electric Strengths. 
Air, ...... 1 
Paraffin when solid, . . . . 5 
Paraffin when liquid, . . . 2’5 
Thus the electric strength of this substance, when in the solid state, 
is to its electric strength when in the liquid state as 2 to 1 . 
\ 
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As an instance of how these, experiments may be made directly 
useful, I may mention that we obtained two samples of liquid 
paraffin from Mr Calderwood to compare their electric strength. 
We found the ratio to be P6. It is, however, extremely difficult to 
effect the comparison unless we have a considerable quantity of each 
specimen. It is best to have a dish of the above form, where we can 
