Whether a witness who is called as an expert has the requisite qualifications and knowledge to enable him to testify, is a preliminary question for the court. The decision of this question is conclusive, unless it appears upon the evidence to have been erroneous, or to have been founded upon some error in law. Nunes v. Perry, 113 Mass. 274, 276. Commonwealth v. Sturtivant, 117 Mass. 122. In the case at bar, no such error is shown; on the contrary, it appears that the witness had no knowledge of the quality of the coal in question, except through the reports of his engineer. Exceptions overruled.