Bursting Strain.—For quick comparative tests there are a number of machines to choose from. The list comprises the Mullen, Southworth, Woolley, Ashcroft, Eddy, and Rehse machines. Testing on the Mullen machine is by hydraulic pressure which is communicated through the medium of glycerine to a rubber diaphragm. The paper is clamped over the diaphragm; the handle of the machine is turned, pressure being exerted until the paper bursts; the reading is given on the gauge in pounds per square inch. The Southworth gives a similar indication, but the fluid is oil, and a steel plunger punctures the paper. The Woolley machine is actuated by a spring and gives a comparative figure. The Ashcroft is a very compact machine, a very small plunger piercing the paper, the dial reading indicating the bursting strain in pounds per square inch. Two machines of this pattern are made, one for thin papers, and the other for papers of ordinary and thick substances. The Eddy machines are screw machines, also made for thick and thin papers, and the result is given in similar terms to other machines. The Rehse machine is a cylindrical machine; pressure is exerted by a spring, and the pressure in pounds is registered on one scale, and from figures given on another scale the stretch can be calculated.
These machines are exceedingly useful for rapid comparisons of papers, the tests being made at the same time. Slight variations in results can be obtained by turning the handles of the machines at varying speeds, but if a uniform rate is maintained, scarcely any other precaution is necessary in their use.
Opacity.—While it is possible to obtain apparatus for exact determination of the degree of opacity in paper, comparison can readily be made in a simple manner. A printed page is covered by pieces of the
