The opinions of witnesses, other than those who are specially qualified by scientific knowledge, to judge of such matters, are not competent evidence as to the soundness or unsoundness of mind of a testator or grantor, at the time of executing a will or deed.

The case of subscribing witnesses to a will or deed, forms an exception to this rule, their opinions being always competent.

(S. C., 13 Barb. 550; 9 N. Y. 371.)