Objection 1. It seems that no liquid can be mingled with the consecrated wine, because everything mingled with another partakes of its quality. But no liquid can share in the quality of the sacramental species, because those accidents are without a subject, as stated above (Article 1). Therefore it seems that no liquid can be mingled with the sacramental species of the wine.
Objection 2. Further, if any kind of liquid be mixed with those species, then some one thing must be the result. But no one thing can result from the liquid, which is a substance, and the sacramental species, which are accidents; nor from the liquid and Christ's blood, which owing to its incorruptibility suffers neither increase nor decrease. Therefore no liquid can be mixed with the consecrated wine.
Objection 3. Further, if any liquid be mixed with the consecrated wine, then that also would appear to be consecrated; just as water added to holy-water becomes holy. But the consecrated wine is truly Christ's blood. Therefore the liquid added would likewise be Christ's blood otherwise than by consecration, which is unbecoming. Therefore no liquid can be mingled with the consecrated wine.
Objection 4. Further, if one of two things be entirely corrupted, there is no mixture (De Gener. i). But if we mix any liquid, it seems that the entire species of the sacramental wine is corrupted, so that the blood of Christ ceases to be beneath it; both because great and little are difference of quantity, and alter it, as white and black cause a difference of color; and because the liquid mixed, as having no obstacle, seems to permeate the whole, and so Christ's blood ceases to be there, since it is not there with any other substance. Consequently, no liquid can be mixed with the consecrated wine.
On the contrary, It is evident to our senses that another liquid can be mixed with the wine after it is consecrated, just as before.
I answer that, The truth of this question is evident from what has been said already. For it was said above (3; 5, ad 2) that the species remaining in this sacrament, as they acquire the manner of being of substance in virtue of the consecration, so likewise do they obtain the mode of acting and of being acted upon, so that they can do or receive whatever their substance could do or receive, were it there present. But it is evident that if the substance of wine were there present, then some other liquid could be mingled with it.
Nevertheless there would be a different effect of such mixing both according to the form and according to the quantity of the liquid. For if sufficient liquid were mixed so as to spread itself all through the wine, then the whole would be a mixed substance. Now what is made up of things mixed is neither of them, but each passes into a third resulting from both: hence it would result that the former wine would remain no longer. But if the liquid added were of another species, for instance, if water were mixed, the species of the wine would be dissolved, and there would be a liquid of another species. But if liquid of the same species were added, of instance, wine with wine, the same species would remain, but the wine would not be the same numerically, as the diversity of the accidents shows: for instance, if one wine were white and the other red.
But if the liquid added were of such minute quantity that it could not permeate the whole, the entire wine would not be mixed, but only part of it, which would not remain the same numerically owing to the blending of extraneous matter: still it would remain the same specifically, not only if a little liquid of the same species were mixed with it, but even if it were of another species, since a drop of water blended with much wine passes into the species of wine (De Gener. i).
Now it is evident that the body and blood of Christ abide in this sacrament so long as the species remain numerically the same, as stated above (4; 76, 6, ad 3); because it is this bread and this wine which is consecrated. Hence, if the liquid of any kind whatsoever added be so much in quantity as to permeate the whole of the consecrated wine, and be mixed with it throughout, the result would be something numerically distinct, and the blood of Christ will remain there no longer. But if the quantity of the liquid added be so slight as not to permeate throughout, but to reach only a part of the species, Christ's blood will cease to be under that part of the consecrated wine, yet will remain under the rest.
Reply to Objection 1. Pope Innocent III in a Decretal writes thus: "The very accidents appear to affect the wine that is added, because, if water is added, it takes the savor of the wine. The result is, then, that the accidents change the subject, just as subject changes accidents; for nature yields to miracle, and power works beyond custom." But this must not be understood as if the same identical accident, which was in the wine previous to consecration, is afterwards in the wine that is added; but such change is the result of action; because the remaining accidents of the wine retain the action of substance, as stated above, and so they act upon the liquid added, by changing it.
Reply to Objection 2. The liquid added to the consecrated wine is in no way mixed with the substance of Christ's blood. Nevertheless it is mixed with the sacramental species, yet so that after such mixing the aforesaid species are corrupted entirely or in part, after the way mentioned above (Article 5), whereby something can be generated from those species. And if they be entirely corrupted, there remains no further question, because the whole will be uniform. But if they be corrupted in part, there will be one dimension according to the continuity of quantity, but not one according to the mode of being, because one part thereof will be without a subject while the other is in a subject; as in a body that is made up of two metals, there will be one body quantitatively, but not one as to the species of the matter.
Reply to Objection 3. As Pope Innocent says in the aforesaid Decretal, "if after the consecration other wine be put in the chalice, it is not changed into the blood, nor is it mingled with the blood, but, mixed with the accidents of the previous wine, it is diffused throughout the body which underlies them, yet without wetting what surrounds it." Now this is to be understood when there is not sufficient mixing of extraneous liquid to cause the blood of Christ to cease to be under the whole; because a thing is said to be "diffused throughout," not because it touches the body of Christ according to its proper dimensions, but according to the sacramental dimensions, under which it is contained. Now it is not the same with holy water, because the blessing works no change in the substance of the water, as the consecration of the wine does.
Reply to Objection 4. Some have held that however slight be the mixing of extraneous liquid, the substance of Christ's blood ceases to be under the whole, and for the reason given above (Objection 4); which, however, is not a cogent one; because "more" or "less" diversify dimensive quantity, not as to its essence, but as to the determination of its measure. In like manner the liquid added can be so small as on that account to be hindered from permeating the whole, and not simply by the dimensions; which, although they are present without a subject, still they are opposed to another liquid, just as substance would be if it were present, according to what was said at the beginning of the article.
Source: Summa Theologica (New Advent)