SCIENTIFIC SUMMARY. 
375 
A new Instilment for the Detection and Measurement of an Inflam- 
mable Gas in Mines was brought before the Physical Society by Mr. E. H. 
Liveing. He noticed that instruments already existing for this object were 
either physical or chemical. Of the former category were Ansell’s and 
Forbes’s: one depending on diffusion; the other on sound-velocity. Of the 
latter were the flame-test, Coquillion’s instrument, and the one now first 
described. 
The principle is as follows : — A mixture of marsh-gas and air, in which 
the former forms less than 5 per cent, is not explosive or capable of continuing 
its own combustion at ordinary temperatures and pressures, because the 
heating value of the marsh-gas is insufficient to raise the large excess of 
atmospheric air to the necessary ignition-temperature. If, however, such 
a mixture is exposed to some sufficiently heated object, especially platinum, 
it will bum in its immediate contact and neighbourhood, and, in so doing, 
add materially to the temperature of the object ; the more so the larger the 
percentage of gas present. 
Two small similar spirals of fine platinum- wire are placed in the same 
circuit from a small magneto-electric machine. One is enclosed in a tube con- 
taining air, the other is a cylinder of wire-gauze ; both furnished with glass 
ends, the latter exposed to the gas to be examined. If the air contain above 
A<r of marsh-gas, the spiral exposed to it increases in brilliancy, and can be 
made to determine the percentage of gas present. This is accomplished by 
placing a wedge-shaped screen between the two spirals and viewing it 
through a side-tube. The screen can be moved towards either end till equal 
illumination is obtained, and an empirical scale can then be read off, giving 
the percentage of gas. 
Certain Effects of Stress on Soft Iron Wires have been studied by Mr. J. 
Ewing, at ToHo, Japan. The wire was Japanese, and annealed. It was 
hung vertically from a strong frame,, and a tank holding 100 kilos, of water 
was attached to its lower end. The weight of the tank was balanced. It 
was circular, and of uniform diameter, the stress thus being proportionate to 
the height of water, as recorded horizontally on a sheet of paper drawn along 
by a float. A pencil traversed the paper at right angles through distances 
proportional to the wire’s elongation. A continuous diagram was thus drawn 
automatically from zero to breaking point. 
When a constant stress was maintained for a considerable time (40^ 
hours), the effect was remarkable. The wire, instead of continuing to 
lengthen at once when the flow of water was resumed, refused to stretch 
further until the stress rose to 40 from 35 kilos. There was in fact a new 
1 limit of elasticity ’ at this point. It broke with 41 kilos. This 1 hardening 
effect ’ depended on the length of time during which the trial is interrupted, 
increasing more rapidly at first than after. If the load were entirely re- 
moved, no considerable lengthening took place till a far higher stress than 
before was reached. Thus an interval of no stress had a hardening effect, 
like one of constant stress. A stress, therefore, produces two effects, (1), a 
gradual viscous elongation, at first rapid, afterwards slow ; (2), a hardening 
effect, also greatest at first. The effects are also perceptible in copper and 
brass, but much less than in soft iron. 
