Fire Surveying and Assessing.
In a second article on this subject which appeared recently in the columns of The Surveyor Mr. Sidney Greenwood, the well known fire-assessor, deals with fires caused by the sun’s rays. At the outset Mr. Greenwood admits that comparatively few outbreaks are really due to the direct action of the sun’s rays, but there are cases where buildings, farms and forests have been levelled to the ground solely through the action of this luminary. The majority of such fires are not, however, attributed to this cause, but the word incendiarism is tacked on to the report, thereby putting aside any further enquiries.
As an instance of what he considers were most probably fires caused by the sun’s rays, Mr. Greenwood mentions the fact that on one day in April the Metropolitan Fire brigade received six calls to furze fires, two of whiclt were on Hampstead Heath. The day in question was about the hottest day of the year, which, he thinks, points to the sun’s heat as the probable origin of the outbreaks. On a common or waste land there are many objects, such as broken bottles of various kinds, which would have the effect of focussing the sun’s rays. This may be so, but we are ourselves inclined to think that a far more probable cause of such fires as those which destroyed the most beautiful part of Hampstead Heath was either the common wax matches carelessly thrown down by smokers or incendiarism by boys out of a love of mischief. One of the fires referred to was certainly due to the latter cause.
“By tracing back from forty or fifty years ago, before sheet glass had become generally used,” continues Mr. Greenwood, “it will be found that the larger number of fires occurred during the hottest part of the day, when, perhaps, all hands were at dinner, thereby allowing the fire to get a firm hold before discovery. From a glance at the windows of Old London it will be seen that the panes were not only thick and small, but in many instances contained what are commonly termed ‘bull’s-eyes,’ which are even to the present day used to adorn the windows of a smith’s shop and the door panels of modern suburban residences. The risk from these windows is very great, focussing, as they do, the rays of the sun precisely as a magnifying glass does. Taking a more scientific view of the subject, it will be seen that out of ioo heat rays glass absorbs ninety and reflects ten. Well-polished silver absorbs only three and reflects ninety seven, while lamp black absorbs all and reflects none. The radiating power of substances being exactly equal to their absorbing powers, therefore good radiators are good absorbers. Substances like glass are equally transparent and diathermanous; air allows the heat of the surl as well as light to pass through it. Dry air does not intercept the heat, so it does not get warmer in consequence of the radiant heat that is passing through it. Amongst liquids bisulphite of carbon is the best representative of this class. Some substances are more transparent than diathermanous. Thus crown glass stops more of the heat than of the light of a sunbeam. Water, although so transparent, is one of the least diathermanous of liquids. Then, again, rock salt, which is less transparent than glass, is much more diathermanous. Iodine, which is opaque to light, readily allows heat to pass through it; and by means of a cell containing iodine dissolved in bisulphide of carbon we may stop all the light from a beam of the sun, or from an electric lamp, and allow a great portion of the heat of the beam to pass on.
“Further, with regard to my theory as to the origin of fires being caused by solar heat, it is a fact that paper and wood can be ignited by intense sunlight produced by convection of the rays or by a lenticular object, such as a bottle or carboy.
“Platinum and quartz have been melted by the sun’s rays in the focus of a plano-convex lens; and, as is well known, Buffon, the celebrated French naturalist, succeeded in setting fire to timber, at a distance of 80 to 90 yards, by concentrating the sun’s rays upon it.”