FIRE-DAMP INDICATOR 395 



Carbonic acid gas is a chemical substance, composed of ono equivalent of carbon 

 and two equivalents of oxygen ; it is a dense gas of a poisonous nature, having a specific 

 gravity 1*521; its density causes it to flow to and accumulate upon the floor of a 

 chamber. In an atmosphere, containing 10 per cent, of carbonic acid, life can be 

 maintained for a short time only, and a candle for a still less time; indeed, Dr. 

 Angus Smith has observed that 3 per cent, is fatal, if the amount of oxygen falls 

 below 18 per cent, in the same atmosphere. 



"We have thus spoken of fire-damp and of carbonic acid, because wo are about to 

 explain Mr. Ansell's proposition for giving warning of the existence of these sub- 

 stances through the agency of diffusion by means of a simple system of telegraphy. 

 Mr. George F. Ansell, late of the Koyal Mint, in a recent lecture on this subject, said : 

 * Possessing this information, I visited some pits, that I might see the precise con- 

 ditions to be met, and I placed before myself the problem, how to make known, by 

 their own agency, the existence of substances so varying as carbonic acid and fire- 

 damp ; it was perfectly manifest that the specific gravity, as specific gravity which 

 had been proposed by others, must fail, because of the dust and currents of air in 

 the pits ; but these thoughts developed the idea that specific gravity, as a diffusive 

 agent, would be the plan, and I have up to this date been unable to improve the first- 

 formed idea, although I have been enabled to greatly simplify the mode of operation. 

 I felt all along that if the existence of dangerous gases could be made known to 

 the master without the man's agency, then the servants would be more particular 

 to observe the laws laid down by the master, and so conduce to the welfare of all.' 



The diffusion of gases may be described as an intermingling of them when placed 

 in such circumstances as to admit of the natural motion of their particles extending 

 itself into space or into other gases ; for this explanation, it is. well to consider that 

 matter in every form, whether solid, liquid, or gaseous, that is liquid in a higher 

 sense, is formed of minute particles, just as the ocean is formed of drops of water. 

 Those minute particles have been called atoms or molecules, and there are reasons for 

 believing that these atoms have motion amongst themselves in either the solid, liquid, 

 or gaseous form of matter. In the case of gases, the law governing one particular 

 motion was eliminated by the late Mr. Thomas Graham, who demonstrated that 

 gases diffuse into each other and into space in the inverse ratio to the square root of 

 their densities ; in other words, a light gas diffuses itself rapidly, while a dense gas 

 diffuses itself slowly into space, or into another gas. 



If we take the following substances, and compare them with atmospheric air, as the 

 standard of specific gravity, they will illustrate our meaning, for 



Specific gravity. Cubic in. Grains. 



Air being taken as of 1*0000 , 100 will weigh 31-0117 



Hydrogen being taken as of 0*0691 2*1400 



Marsh gas being taken as of 0*5590 17*4100 



Carbonic acid being taken as of 1*5240 47*2600 



The barometer registering 30 inches, and the thermometer 60 P. If, therefore, a 

 vessel of a given capacity be filled with each of these different gases under precisely 

 similar circumstances, and weighed carefully, the variations of weight would be as 

 indicated above. 



If, then, we imagine the containing vessel to be made of such a substance as will 

 admit of diffusion, we should find that the gas would diffuse out of that vessel in a 

 period of time relative to its specific gravity: hydrogen most quickly, marsh gas 

 next in its place, air following, and lastly carbonic acid ; each gas being replaced by 

 its diffusion-equivalent of air ; but in the case of air, although diffusion would take 

 place, its amount would not be registered, because air would replace air, volume for 

 volume. 



To demonstrate this with one experiment, it will be sufficient to fill a glass tube, 

 whose one end is closed by a plug of plaster of Paris, with coal-gas, and then to 

 place its lower end in water, when the water will gradually rise in the tube, because 

 the gas diffuses out into space, leaving a partial vacuum, while the atmosphere presses 

 the water into that space. 



But we find that the whole space is not exhausted, yet there is no gas remaining ; 

 for while the gas has diffused out, air has diffused into the tube (the gases have 

 passed each other within the interstices of the plaster of Paris), and thus we come to 

 the relative movement of gases under the circumstances of diffusion, or endosmose and 

 exosmose, as it was formerly called. 



The same gas, diffusing through different substances, occupies varying times, 

 being quicker through unglazed pottery ware of Wedgwood than through Sicilian 

 marble. The force or power of diffusion is considerable, as will be evident if the 

 little apparatus about to be described be placed in an atmosphere of coal-gas. The 



