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. 
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 1*6. 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 
