[425]. Under the head of material things we must class the qualities of matter, so far as they are capable in law of being in themselves the objects of rights. The qualities which thus admit of separate legal appropriation are two in number, namely force and space. Electricity is in law a chattel, which can be owned, sold, stolen, and otherwise rightfully and wrongfully dealt with. 45 & 46 Vict. c. 56, s. 23. Definite portions of empty space are capable of appropriation and ownership, no less than the material objects with which other portions of space are filled. The interior of my house is as much mine as are the walls and the roof. It is commonly said that the owner of land owns also the space above the surface usque ad coelum. Whether this is truly so is a doubtful point as the law stands, but there is no theoretical difficulty in allowing the validity of such a claim to the ownership of empty space.
[426]. The distinction formerly noticed by us (§ 88) between corporeal and incorporeal things must not be confounded with the present distinction between material and immaterial things. The latter is a logical distinction, but the former is a mere artifice of speech. An incorporeal thing is a kind of right, namely any right which is not identified with some material thing which is its subject-matter. An immaterial thing is not a right but the subject-matter of one. It is any subject-matter of a right except a material object.
[427]. Supra § 83.
[428]. Supra § 90.
[429]. Encumbrances are not confined to the law of property, but pertain to the law of obligations also. Choses in action may be mortgaged, settled in trust, or otherwise made the subject-matter of jura in re aliena, no less than land and chattels. Much, therefore, of what is to be said here touching the nature of the different forms of encumbrance is equally applicable to the law of rights in personam.
[430]. Possession by way of security only, e.g., a pledge, is differentiated by its purpose, however, and falls within the class of securities, not within that of leases.
[431]. Beardman v. Wilson, L. R. 4 C. P. 57.
[432]. An example of a lease in perpetuity is the emphyteusis of Roman law. In consequence of its perpetuity the Roman lawyers were divided in opinion as to the true position of the tenant or emphyteuta, some regarding him as an owner and others as an encumbrancer. The law was finally settled in the latter sense. Just. Inst. III. 24. 3.
[433]. The term servitude (servitus) is derived from Roman law, and has scarcely succeeded in obtaining recognition as a technical term of English law. It is better, however, than the English easement, inasmuch as easements are in the strict sense only one class of servitudes as above defined.
[434]. It is only over land that servitudes can exist. Land is of such a nature as to admit readily of non-possessory uses, whereas the use of a chattel usually involves the possession of it for the time being, however brief that time may be. The non-possessory use of chattels, even when it exists, is not recognised by the law as an encumbrance of the ownership, so as to run with it into the hands of assignees.