Firth. — On Fires in Coal-ships. 109 



Shipmasters' Society, London, and to which I am much 

 indebted.) 



Pumice Insulation. 



On the question of the prevention of heat passing from 

 stoke-holes or boilers to the coal-bunkers, instead of Profes- 

 sor Lewes's suggestion of filling the double iron bulkheads with 

 sea-water, or filling them with air, as Captain Froud suggests, 

 I would propose their being filled with calcined pumice ; this 

 material, with its imiumerable air-cells, being indestructible 

 and incombustible, and being one of the best and safest non-con- 

 ductors of heat hitherto discovered. Captain Frond's sugges- 

 tion of filling the bulkheads with air is inadmissible, because 

 air facilitates radiation and circulation, under the influence of 

 which the interior of the bulkheads would be heated and the 

 air become a conductor of heat, and of no value. It is only 

 when air is confined in minute cells, as in pumice, that it is 

 a first-class non-conductor of heat. 



Ventilation. 



The ventilation of coal-cargoes by means of iron or wood 

 tubes, if there are no cones of small coal and slack under the 

 hatchways, I consider unnecessary, and if cones of coal be 

 formed in the hold these tubes often do more harm than 

 good. If no cones are formed, surface ventilation by the 

 removal of the hatches occasionally in fine weather will 

 secure all the ventilation necessary. 



Causes of Explosions. 



In a great many of the fires of coal-cargoes explosions of 

 a severe and dangerous character often occur. It is an error 

 to suppose that great quantities of this explosive gas (com- 

 monly known as "marsh-gas ") is generated and thrown off 

 by coal in what ought to be its normal condition in a ship's 

 hold — namely, large and round coal. When in that condi- 

 tion, with a normal temperature of 75° Fahr., only harmless 

 quantities of marsh-gas will be found in the hold. This dan- 

 gerously explosive gas is produced by heat, as may be seen at 

 any gasworks, where large quantities of large and round coal 

 are kept in bulk for considerable periods without developing 

 heat. But when this coal is thrown into heated gas-retorts 

 large quantities of gas are at once thrown off. So it will be 

 in a ship's hold ; if the coal is mainly large and round little or 

 no gas will be evolved, and if no heat-creators in the shape of 

 cones of small coal and slack (which are practically gas- 

 retorts) be allowed to be formed in the ships' holds. It is 

 these heated cones which generate and throw off large quanti- 

 ties of marsh-gas, and which cones, I have no hesitation in 



