The city of Boston offered a very large reward for the detection and conviction of any person who might be guilty of feloniously setting fire to any building, &c. This offer of a reward was cautious and guarded. It contemplated that a person should be guilty, should be detected, and should be convicted of a felonious burning. Perhaps it is difficult to ascertain, as the law now stands, what was meant precisely by the word “ felonious; ” but it is manifest, from the magnitude of the reward, that it contemplated an incendiary act of an aggravated character.

No evidence was offered to prove the guilt of the person charged, except the conviction. The plaintiffs, contended, that this was conclusive, and objected to the admission of any other evidence, and the court so ruled. On consideration, the court are of opinion, that although the record of conviction was competent evidence, because conviction, as well as guilt and detection, was to be proved, yet it was not conclusive. It is impossible to ascertain from this record, whether the offence was of a high and aggravated nature, or *407simply the placing of a burning coal on some part of the building, by which the slightest actual combustion occurred. The charge against the defendants was, that, in the daytime, in the month of June, a certain building, formerly a shop, intended for a dwelling-house, and in the process of being fitted up for that purpose, they did wilfully and maliciously set fire to and burn. The slightest intentional ignition of the building, in whatever part of it, and however slight, would be sufficient to sustain this charge. It is impossible for the court to perceive, and there is no competent evidence to the jury to show, that this was a felonious burning.

But, upon the more general ground, we are strongly inclined to the opinion, that in no case is the conviction in a criminal case conclusive, because it may have been obtained upon the testimony of the plaintiff, or other persons interested. It is not the fact, that the verdict was or was not obtained upon the testimony of the plaintiff, or of some other party interested, that renders the conviction incompetent evidence, but it is rejected because it is res inter alios ; and one reason why any person can be received as a witness in a criminal case is, that the conviction or acquittal cannot be used elsewhere, and between other parties. The admission, in a civil action, of a conviction on an indictment founded on a plea of guilty, is not an exception to this rule. That is received, not as a judicial act, having the force and effect of a judgment; but as a solemn confession of the very matter charged in the civil action. 1 Greenl. Ev. § 537.

Verdict set aside, and a new trial ordered.