Objection 1. It seems that the species which remain in this sacrament cannot affect external objects. For it is proved in Phys. vii, that forms which are in matter are produced by forms that are in matter, but not from forms which are without matter, because like makes like. But the sacramental species are species without matter, since they remain without a subject, as is evident from what was said above (Article 1). Therefore they cannot affect other matter by producing any form in it.
Objection 2. Further, when the action of the principal agent ceases, then the action of the instrument must cease, as when the carpenter rests, the hammer is moved no longer. But all accidental forms act instrumentally in virtue of the substantial form as the principal agent. Therefore, since the substantial form of the bread and wine does not remain in this sacrament, as was shown above (Question 75, Article 6), it seems that the accidental forms which remain cannot act so as to change external matter.
Objection 3. Further, nothing acts outside its species, because an effect cannot surpass its cause. But all the sacramental species are accidents. Therefore they cannot change external matter, at least as to a substantial form.
On the contrary, If they could not change external bodies, they could not be felt; for a thing is felt from the senses being changed by a sensible thing, as is said in De Anima ii.
I answer that, Because everything acts in so far as it is an actual being, the consequence is that everything stands in the same relation to action as it does to being. Therefore, because, according to what was said above (Article 1), it is an effect of the Divine power that the sacramental species continue in the being which they had when the substance of the bread and wine was present, it follows that they continue in their action. Consequently they retain every action which they had while the substance of the bread and wine remained, now that the substance of the bread and wine has passed into the body and blood of Christ. Hence there is no doubt but that they can change external bodies.
Reply to Objection 1. The sacramental species, although they are forms existing without matter, still retain the same being which they had before in matter, and therefore as to their being they are like forms which are in matter.
Reply to Objection 2. The action of an accidental form depends upon the action of a substantial form in the same way as the being of accident depends upon the being of substance; and therefore, as it is an effect of Divine power that the sacramental species exist without substance, so is it an effect of Divine power that they can act without a substantial form, because every action of a substantial or accidental form depends upon God as the first agent.
Reply to Objection 3. The change which terminates in a substantial form is not effected by a substantial form directly, but by means of the active and passive qualities, which act in virtue of the substantial form. But by Divine power this instrumental energy is retained in the sacramental species, just as it was before: and consequently their action can be directed to a substantial form instrumentally, just in the same way as anything can act outside its species, not as by its own power, but by the power of the chief agent.
Source: Summa Theologica (New Advent)