_They Must Obey the Summons and Appear and be Sworn. In General they need Not Give their Opinions unless Duly Compensated._—An expert witness is brought into court like an ordinary witness by the usual process of the court. This process is, under the American system, an ordinary subpœna, and, being process of the court, whether or not he has been paid or promised compensation for giving his opinion he must obey the process to the extent at least of appearing in court when called, to be sworn. Interesting questions have been raised as to his obedience to the subpœna to the extent of testifying when he has not been compensated. It has been argued, and the argument is sustained by the decisions of courts of high authority in some States, that his knowledge and skill, acquired by study and by experience, is his property, of which he cannot be deprived without just compensation, under his constitutional rights guaranteed to him by the organic law of this country. On the other hand, in some other States it has been held that he is so far a necessary part of the judicial system that he may be called upon to give the results of his experience, knowledge, and skill forming his opinion, without payment other than the ordinary compensation to witnesses. It is believed, however, that the better opinion is the former; that he does not stand on the same footing as an ordinary witness, whose province it is to testify solely to matters of observation of fact, but that he stands in the position of one who has something to give; something to impart in the way of knowledge or experience, which is his property as much as any other thing movable or immovable of which he is possessed.