“As in all the following visits to the Most Blessed Sacrament a spiritual communion is recommended, it will be well to explain what it is, and the great advantages which result from its practice. A spiritual communion, according to St. Thomas, consists in an ardent desire to receive Jesus in the Most Holy Sacrament, and in lovingly embracing Him as if we had actually received Him. How pleasing these spiritual communions are to God, and the many graces which He bestows through their means, was manifested by our Lord Himself to Sister Paula Maresca, the foundress of the convent of St. Catherine of Sienna in Naples, when (as it is related in her life) He showed her two precious vessels, the one of gold, the other of silver; He then told her that in the gold vessel He preserved her sacramental communions, and in the silver one her spiritual communions. He also told Blessed Jane of the Cross, that each time that she communicated spiritually she received a grace of the same kind as the one which she received when she really communicated. Above all, it will suffice us to know that the holy Council of Trent greatly praises spiritual communions, and encourages the faithful to their practice. 

　Hence all devout souls are accustomed often to practise this holy exercise of spiritual communion. Blessed Agatha of the Cross did so two hundred times a day. And Father Peter Faber, the first companion of St. Ignatius, used to say that it was of the highest utility to make spiritual communions, in order to receive the sacramental communion we ll.”   St. Alphonsus de Liguori  (Taken from his work VISITS TO THE MOST HOLY SACRAMENT AND THE BLESSED VIRGIN MARY. translated into the English by THE REV. R. A. COFFIN, C.SS.R)