1865.] Mining, Mineralogy, and Metallurgy. 483 



called a diffusionieter, has obtained an exact measurement of the rate at 

 which this interchange takes place. He found that the diffusion of 

 gases through porous septa is regulated by the same law as when they 

 communicate freely with one another, namely, that the relative diffusi- 

 bilities are inversely as the square roots of the densities. 



A very beautiful application of the Diifusion of Gases has been re- 

 cently patented by Mr. G. F. Ansell, of the Eoyal Mint. In our coal 

 mines, there are not unfrequently, evolved from the coal beds, consider- 

 able quantities of light carburetted hydrogen gas. This gas mixing 

 with atmospheric air, forms the explosive fire damp, from the ignition 

 of which the most disastrous consequences too frequently ensue. Mr. 

 Ansell's object has been to construct an instrument which should in- 

 dicate the presence of this gas in any part of the coal mine— give 

 warnings of its presence — and, indeed, measure the percentage quan- 

 tity in which it exists relatively to atmospheric air. We have given 

 drawings of some forms which Mr. Ansell's instruments have taken, 

 and a description of those will sufficiently explain their action under 

 the influence of the diffusing gas. 



Carburetted hydrogen gas, — either heavy, as ordinary coal gas, or 

 light, as marsh gas or coal-mine gas, though lighter than air, — diffuses 

 itself readily through air. Upon this is founded the method for 

 detecting its presence. Fig. la is a small balloon of india-rubber 

 filled with atmospheric air, bound around the middle with a band of 

 linen, to prevent lateral expansion. This is placed under the lever (b), 

 resting on a stand at (/), where there is a screw to adjust the bag of 

 air to a proper height. When adjusted, the balloon presses lightly 

 upon the lever (b) already referred to. If this arrangement is placed 

 in an atmosphere containing but a small percentage of carburetted 

 hydrogen, this gas passes rapidly through the india-rubber by dif- 

 fusion, and, of course, expands the balloon. In expanding this, the 

 lever is raised ; and by the arrangement shown at (c), the rachet wheel 

 is released, and the cord and weight (d) liberated, and in the fall of 

 the latter down the pillar, a bell is rapidly and repeatedly rung. By 

 the same form of apparatus, connection may be made, or broken, with 

 a voltaic battery, or a magneto-electric machine put in motion, and a 

 telegraphic signal given at the surface of the mine, either in the office 

 or in any convenient place. 



Fig. 2 is another form of the instrument. A cup of mercury is 

 inclosed in a cell of porous earthenware, through the top of which a 

 tube passes to the bottom of the mercury. On the surface of the 

 mercury, in the tube, is a floating weight attached to a string, which 

 is carried over a pulley, and, by its rise or fall, moves an index upon a 

 dial, precisely in the same manner as in the ordinary wheel barometer. 

 The gas, even when it exists in the smallest quantity, passes into 

 the porous cell, and thus increasing the volume of the inclosed air, it 

 presses the mercury up into the tube, lifting the float, and thus 

 moving the index. This arrangement may be made to ring a bell, or 

 to establish electro-telegraphic communication. 



Fig. 3 is a glass U tube, holding a little mercury or coloured 

 water. This is fixed upon a board, on which is a scale graduated by 



2 l2 



