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_. * * * * * 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. Each student may have his own desk and apparatus or he may be required to pass from desk to desk. The latter method is preferable. When a store room is used the services of a man are required to distribute and afterward to collect. If the apparatus is permanently distributed, a large room is necessary, but the labor of collecting and distributing is done away with. There are certain general experiments intended to show the use of measuring instruments which all students must perform. To illustrate the use of the indicator I have selected an elementary class, although the instrument is equally applicable to all classes of experiments.