I answer that, As stated above (Q. 7, A. 1; Q. 18, A. 5, ad 4; AA. 10, 11), when we were treating of circumstances, a circumstance, as such, is an accident of the moral act:  and yet a circumstance may happen to be taken as the specific difference of a moral act, and then it loses its nature of circumstance, and constitutes the species of the moral act.  This happens in sins when a circumstance adds the deformity of another genus; thus when a man has knowledge of another woman than his wife, the deformity of his act is opposed to chastity; but if this other be another man’s wife, there is an additional deformity opposed to justice which forbids one to take what belongs to another; and accordingly this circumstance constitutes a new species of sin known as adultery.

It is, however, impossible for a circumstance to make a venial sin become mortal, unless it adds the deformity of another species.  For it has been stated above (A. 1) that the deformity of a venial sin consists in a disorder affecting things that are referred to the end, whereas the deformity of a mortal sin consists in a disorder about the last end.  Consequently it is evident that a circumstance cannot make a venial sin to be mortal, so long as it remains a circumstance, but only when it transfers the sin to another species, and becomes, as it were, the specific difference of the moral act.

Reply Obj. 1:  Length of time is not a circumstance that draws a sin to another species, nor is frequency or custom, except perhaps by something accidental supervening.  For an action does not acquire a new species through being repeated or prolonged, unless by chance something supervene in the repeated or prolonged act to change its species, e.g. disobedience, contempt, or the like.

We must therefore reply to the objection by saying that since anger is a movement of the soul tending to the hurt of one’s neighbor, if the angry movement tend to a hurt which is a mortal sin generically, such as murder or robbery, that anger will be a mortal sin generically:  and if it be a venial sin, this will be due to the imperfection of the act, in so far as it is a sudden movement of the sensuality:  whereas, if it last a long time, it returns to its generic nature, through the consent of reason.  If, on the other hand, the hurt to which the angry movement tends, is a sin generically venial, for instance, if a man be angry with someone, so as to wish to say some trifling word in jest that would hurt him a little, the anger will not be mortal sin, however long it last, unless perhaps accidentally; for instance, if it were to give rise to great scandal or something of the kind.