In the case of a large tree in which the trunk is quite hollow, usually with a basal and top opening, the amount of cement required to fill the cavity precludes the possibility of it being employed on the score of economy, but where the hollow only extends for a comparatively short distance upwards and inwards the composition is to be recommended. When a large amount of material is required to fill a hollow stem, clean bricks broken to about the size of a golf ball may be used, but they must be packed firmly and the surface, wherever it comes in contact with the weather, glazed over with cement in order to effectually shut out dampness. Asphalt has of late years been employed in the filling of hollow stems, and is to be recommended on account of its elasticity and lasting properties. Sometimes, the interior of a large, hollow tree stem is strengthened by crossbeams of timber, any holes which extend to the outside being carefully covered with sheet lead or zinc so as to prevent the ingress of water. A sheet of lead or zinc is, however, only a superficial remedy and should be dispensed with where filling the cavity is at all practicable. Lead, in some cases, is preferable to zinc for covering wounds in trees, as it fits into position more readily and is easily fixed. The treatment of hollow stems without filling the cavity is in certain instances quite permissible, and can be carried out at a comparatively small cost. Broadly speaking, this method consists in cleaning out all decayed and decaying matter, painting the interior with carbolineum or tar, and, where possible, bracing the trunk from the interior by means of stout crossbars of wood and covering surface holes with lead, zinc or sheet copper. When a tree trunk is hollow from top to bottom this method is to be recommended, and stout struts placed within the cavity at various of the weaker points will go far towards preserving many an aged specimen. When the cavity in a hollow stem is so large that a person can enter it, the work of either filling with a suitable composition or supporting with wooden struts is greatly simplified. Recent experiments have proved the value of a mixture of sawdust and asphalt for filling cavities. It is particularly valuable in cases where concrete is too rigid and unyielding, such as in dealing with trunks and branches that are apt to be swayed about in stormy weather. The materials are dry sawdust, that of Oak, Chestnut and Beech being preferable, and solid asphalt derived from the refining of petroleum, which is at present in use for filling the interstices of street pavements in various parts of the metropolis.