506 INVESTIGATIONS RELATIVE TO ILLUMINATING MATERIALS. 



iron to protect the floor. On each of these was then dropped an ordi- 

 nary friction match in a state of ignition. They both broke instantly 

 into flames, which soon entirely consumed the cloth, although but little 

 air could obtain access to its under side, and notwithstanding the good 

 conducting power of the tinned iron. 



In a similar exi)eriment made with the same cloth saturated with 

 lard-oil the cloth did not take fire when a lighted match was dropped 

 upon it. Two cotton cloths the same size were saturated, one with lard- 

 oil, the other with petroleum, and lighted at the same time. The petro- 

 leum cloth was consumed in 1 minute 23 seconds; the lard cloth in 5 

 minutes. 



To render these experiments more strikingly applicable to cases of 

 accident which might occur in a light-house, fl piece of cotton cloth 

 about 2 feet square, which had been used to wipe the table on which 

 kerosene had been spilled, was crumpled up into the condition of an 

 ordinary dish-cloth and thrown into a corner of the room. When a 

 lighted match was dropped on this it • instantly took fire and burned 

 with a fierceness truly alarming. 



These experiments are important in establishing the fact that oils 

 which are commonly sold as entirely free from danger are not really so. 

 They may be safe from explosions at ordinary temperatures, and in this 

 respect are to be preferred to the lighter oils ; but when spread over a 

 large surface they burn with greater intensity, even, as we have seen, 

 on a surface of ice. Indeed, the results are so striking it might be well 

 to repeat them in the presence of every light-house keeper, in order to 

 impress him with an idea of the danger which might be apprehended in 

 spilling the oil over his clothes or in carelessly dropping his matches on 

 cloths which had been used in cleaning the apparatus. • 



Among the peculiar properties of mineral oil is its great surface- 

 attraction or power of adhering and spreading on other surfaces, as well 

 as ascending wicks to a much greater altitude than other oils. This 

 property is recognized by the housekeeper who finds the exterior of the 

 lamp covered with a film of oil shortly after it has been subjected to a 

 thorough cleansing. It rises along the interior surface of the lamp and 

 spreads over the outside. On account of this property it can be freely 

 burned in lamps of which the fountain is at a considerable distance 

 below the flame, and in which no overflow is required to produce a bril- 

 liant combustion. 



A series of experiments was next made with regard to the burning 

 qualities of mineral oils of diflerent densities, from which it was inferred 

 that the lighter oils in lamps of the fourth order gave a greater amount 

 of illumination than the heavier oils, and, furthermore, that the latter 

 charge the wick more than the former, from which it would appear that, 

 in using mineral oil, while safety should be the prominent consideration 

 on the one hand, in the choice of the material, regard must be had on 

 the other to the illuminating power. 



In regard to the relative photometric power of lamps of the same 



