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 thinly fluid mass, the freshly coated surface was strewn with sand.  The most volatile portion of the tar evaporated soon, whereby the coating became thicker and finally dried.  The bad properties of the coal tar, pointed out elsewhere, made it very unsuitable even for this purpose, and experiments were instituted to compound mixtures, by adding other ingredients to the tar, that should more fully comply with its function.  It may be said in general that the coating masses for roofs can be divided into two classes:  either as lacquers or as cements.  To the former may be classed those of a fairly thinly fluid consistency, and which contain volatile oils in such quantities that they will dry quickly.  Cements are those of a thickly fluid consistency, and are rendered thus fluid by heating.  It is not necessary that the coating applied should harden quickly, as it assumes soon after its application a firmness sufficient to prevent it from running off the roof.  Coal tar is to be classed among lacquers.  If it has been liberated by distillation from the volatile oils, it is made better suited for the purpose than the ordinary kind.  The mass contains much more asphaltum, and after drying, which takes place soon, it leaves a far thicker layer upon the roof surface, while the pores, which had formed in the roofing paper consequent on drying, are better filled up.  Nevertheless, the distilled tar also has retained the property of drying with time into a hard, vitreous mass, and ultimately to be destroyed by decomposition.—­The Roofer.

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A PHYSICAL LABORATORY INDICATOR.

The difficulties attending the management of a physical laboratory are much greater than those of a chemical one.  The cause of this lies in the fact that in the latter the apparatus is less complicated and the pieces less varied.  Any contrivance that will reduce the labor and worry connected with the running of a laboratory is valuable.

A physical laboratory may be arranged in several ways.  The apparatus may be kept in a store room and such as is needed may be given to the student each day and removed after the experiments are performed; or the apparatus for each experiment or system of experiments may be kept in a fixed place in the laboratory ready for assembling; for certain experiments the apparatus may be kept in a fixed place in the laboratory and permanently arranged for service.