The instrumentalities through which commerce is carried on between the States and with foreign countries may be taxed by the States as property to the extent that such instrumentalities are within the several territories of the States so taxing them. Thus buildings used for freight and passenger stations and for offices, roadbeds, rails, machine shops, etc., may be taxed by the States in which they are situated, so long as the tax is a general property tax and not one laid upon them specially, nor at a special rate because of their employment in interstate commerce. In determining", however, the value of these properties, the important principle has been laid down that in estimating the value of the property within the State, of a company doing business in several States, the entire property may be treated as a unit and its value in use as such determined, and the value of the part of the property in the particular State estimated as bearing the same proportion to the whole property as the amount of the business done held by the federal Supreme Court itself that a tax on the value of the capital stock of a corporation is a tax on the property in which that capital is invested, and in consequence that no tax can thus be levied which includes property that is otherwise exempt, the court held in the case at bar that the coal actually situated outside of Pennsylvania at the time of the assessment might not be included in the appraisement for purposes of taxation of the capital stock of the plaintiff domestic corporation. "We regard," said the court, "this tax as, in substance and in fact, though not in form, a tax specifically levied upon the property of the corporation, and part of that property is outside and beyond the jurisdiction of the State which thus assumes to tax it."

The doctrine declared in Union Refrigerator Transit Co. v. Kentucky (199 U. S. 194; 26 Sup. Ct. Rep. 36; 50 L. ed. 150) and the two prior cases upon which that case was rested. - Louisville & Jeffersonville Ferry Co. v. Kentucky (188 U. S. 385; 23 Sup. Ct. Rep. 4G3; 47 L. ed. 513) and D., L. & W. R. R. Co. v. Pennsylvania (198 U. S. 341: 25 Sup. Ct. Rep. 669; 49 L. ed. 1077) is a recent doctrine. Until these cases were decided the doctrine was generally held and acted upon in many of the States that all personal property, tangible as well as intangible, wherever situated, might be taxed at the domicile of the owner. In Coe v. Errol (116 U. S. 517; 0 Sup. Ct. Rep. 475; 29 L. ed. 715), decided in 1886. the court say, without qualification, "If the owner of personal property within a State resides in another State which taxes him for that property as part of his general estate attached to his person, this action of the latter State does not in the least affect the right of the State in which the property is situated to tax it also."