Different classification of the same commodity according to the use to which it may be put, is evidently an attempt to grade according to value rather than cost of service. Automobile parts may come in from the wheel-maker at second-class rates, but when they go out to jobbing houses they are rated three times first class.[345] A number of cases of this sort have come before the Commission. Shall cow peas, for example, be classed with corn and oats as agricultural products in one case, while according them a rating with commercial fertilizers in another, inasmuch as they may become an active agent in nitrogenizing soil?[346] More recently the Commission has declined to recognize the validity of classification on this basis. Thus brick is always to be charged the same without regard to whether it is for fire, building or paving purposes.[347] Unusually low rates for steam coal used by carriers and open only to certain shippers for this or other particular purposes, likewise have been forbidden.[348] The carriers have attempted to distinguish in grade between dried fruit and raisins. For the two industries call for relatively different protection against old-established competitors.[349] As actually effected in practice, classification of freight seems to have been largely empirical--the result of long experience in sympathetically feeling the pulse of the business community. In the main, despite their denial of the validity of cost as an element in rate making, traffic managers and the Interstate Commerce Commission seem to have been swayed more commonly by this consideration in the make-up of schedules. Nevertheless, charging what the traffic will bear, as a principle, will suffice alone to explain many of the details of classification now in force. Rates have been adjusted so as to secure the largest amount of business possible at the highest rate compatible with that volume. In other words, traffic managers have been mainly influenced by the consideration well stated by a witness before the United States Industrial Commission: that, "a freight tariff is made as it is, not because it ought to be that, but because it must be that." The procedure of classification committees seems, in other words, to have been mainly based upon considerations of revenue, and that, too, without any very positive evidence as to details.[350] Rule-of-thumb experience, therefore, is mainly represented in classifications of the present time; that is to say, an adjustment of freight rates upon different commodities to suit the commercial conditions which have happened to prevail at any given time. All of which emphasizes still further the need of scientific revision of these most important schedules, preferably by the carriers themselves, but by public authority if commercial inertia be too powerful to be overcome.