In giving the instructions to the jury, I stated to them that if the evidence for the commonwealth was believed, it established a case of murder of the first degree, unless they thought proper to acquit entirely upon the ground of _insanity_. The species of insanity relied upon by the prisoner’s counsel, was that denominated _monomania_. This exists where there is a _delusion_ on _one_ or a _small number_ of subjects, which no course of reasoning or force of evidence can remove. Every man, of mature age, is _presumed_ to possess a sound mind until the _contrary_ appears. To establish this kind of insanity, _delusion_ must be shown to exist on one subject, or on some small number of subjects. It was stated to the jury, that the court could perceive no sufficient evidence of _delusion_ or hallucination on any subject to establish the existence of _monomania_; still, if the jury believed that the prisoner was, at the time of committing the act charged, “incapable of judging between right and wrong, and did not know that he was committing an offence against the laws of God and man,” it would be their duty to acquit; and if they did so, it would be necessary to specify in their verdict the ground of acquittal, in accordance with the act of assembly of 13th June, 1836. But (continued the court) if any insanity exists in this case, it is of that description denominated MORAL INSANITY. This _arises from the existence of some of the natural propensities in such violence, that it is impossible not to yield to them_. It bears a striking resemblance to _vice_, which is said to consist in “an undue excitement of the passions and will, and in their irregular or crooked actions leading to crime.” It is therefore to be received with the utmost scrutiny. It is not _generally_ admitted in legal tribunals as a species of insanity which relieves from responsibility for crime, and it ought _never_ to be admitted as a defence until it is shown that these propensities exist in such violence as to subjugate the intellect, control the will, and render it impossible for the party to do otherwise than yield. _Where its existence is thus fully established, this species of insanity, like every other, relieves from accountability to human laws._ But this state of mind is not to be presumed without evidence; nor does it usually occur without some premonitory symptoms indicating its approach. On this branch of the case the prisoner’s counsel have introduced the testimony of Mr. O. S. Fowler, one of the most distinguished phrenologists in the United States. The science of PHRENOLOGY, or rather CRANIOSCOPY, has not yet been brought to such a state of perfection and certainty as to be received and relied upon in courts of justice. Small deviations in the scull from