must have been washed by the rain water out of the paper. But since
such a solid substance, easily soluble in water, is contained neither
in the fresh roofing paper nor in the coal tar, the only deduction is
that it must have arisen by the decomposition of the tar, in
consequence of the operation of the oxygen. The lime comes from the
coating substance of the roof, for which tar mixed with coal pitch was
used. The latter was fused with carbonate of lime. These analyses
furthermore show that the formation of the organic acid easily soluble
in water depends upon the season; and that a larger quantity of it is
generated in warm, sunny weather than in cold, without sunshine. This
peculiarity of the solid, resinous constituents of the coal tar, to be
by the operation of the atmospheric oxygen altered into such products
that are readily soluble in water, makes the tar very unsuitable as a
saturative substance for a roofing paper. How rapidly a paper roof can
be ruined by the generation of this injurious organic acid will be
seen from the following calculation: Let us suppose that an average
of 132 gallons of rain water falls upon ten square feet roof surface
per year, and that the arithmetical mean 0.932 of the largest (1.680)
and smallest number (0.184) be the quantity of the soluble brown
substance which on an average is dissolved in one quart of rain water;
hence from ten square feet of roof surface are rinsed away with the
rain water per year 466 grammes of the soluble decomposition products
of the tar. The oxidation process will not always occur as intensely
as by a paper roof, ten years old and painted two years ago, which
instigated above described experiment. As long as the roofing paper is
fresh and less porous, especially if the occurring pores are filled
and closed again by repeated coatings, oxidation will take place far
less rapidly. Besides this, the protective coating applied to the roof
surface is exposed most to this oxidation process. Even by assuming
this constantly progressive destructive action of the oxygen on the
roofing paper to be much less than above stated, we can readily
imagine that it must be quite large. If it is desired to produce a
material free of faults, it is first of all indispensable that
unobjectionable raw material be procured. Coal tar was formerly used
almost exclusively for the coating of a roof. It was heated and
applied hot upon the surface. In order to avoid the running off of the