52 
Proceedings of the Royal Society 
Lave broad plates, the lower one slightly raised above the glass 
bottom, and the upper one well immersed in the liquid. The 
former of these conditions helps to prevent solid particles getting in 
between the plates, and the latter prevents the rising of the liquid 
up the stem, and consequent splashing about of liquid particles. 
Discharge through Paraffin Vapour . — We put the discharging 
vessel, with a quantity of one of the pure liquid'paraffins, inside the 
receiver of an air-pump, exhausted the air, and allowed the paraffin 
vapour to accumulate. When the barometer-gauge indicated 50 
mm. pressure, the distance being ^ inch, we took sparks through the 
vapour. The spark was of a broad section, green at either end, but 
of a deep violet between. When a quantity of air was let in, white 
jagged sparks were observed in the midst of the coloured spark. 
From the readings obtained at 50 mm. pressure, we infer that this 
paraffin vapour is 1*7 times as strong as air. 
5. Laboratory Note. By Professor Tait. 
Last autumn I received from Mr Maclachlan of Lower Green, 
Mitcham, some specimens of india-rubber tape which had been for 
several years wound, under considerable tension, helically round 
copper wire. At ordinary temperatures, after being peeled off, the 
material shows no tendency to contract ; but Mr Maclachlan found 
that in hot water it almost instantly resumes its original dimensions. 
I have recently reproduced almost exactly the same results by 
stretching sheet india-rubber, slightly warmed, helically round glass 
tubes, and immersing it for a short time in a freezing mixture. 
Some of the specimens thus produced in a few minutes compared 
favourably in their after behaviour in hot water with those of Mr 
Maclachlan. 
Even without the use of a freezing mixture the effect may be 
produced, though not so perfectly, by drawing out the heated india- 
rubber to the point at which its intensibility begins to diminish 
very rapidly. If it be held for a few seconds in that state of exten- 
sion, it shows very little tendency to contract till it is immersed 
again in hot water. Then it is instantly reduced to one-fourth or 
one-fifth of its previous length, but remains permanently stretched 
to three or four times its original length. This operation may be 
