I've always been intrigued by this custom:
It is also well known and is the plain fact, seen even unto this day, that when the blood of St. Januarius, kept dried up in a small glass phial, is put in sight of the head of the same martyr, it is wont to melt and bubble in a very strange way, as though it had but freshly been shed."
It is especially this miracle of the liquefaction which has given celebrity to the name of Januarius, and to this we turn our attention. Let it at once be said that the supposition of any trick or deliberate imposture is out of the question, as candid opponents are now willing to admit. For more than four hundred years this liquefaction has taken place at frequent intervals. If it were a trick it would be necessary to admit that all the archbishops of Naples, and that countless ecclesiastics eminent for their learning and often for their great sanctity, were accomplices in the fraud, as also a number of secular officials; for the relic is so guarded that its exposition requires the concurrence of both civil and ecclesiastical authority. Further, in all these four hundred years, no one of the many who, upon the supposition of such a trick, must necessarily have been in the secret, has made any revelation or disclosed how the apparent miracle is worked. Strong indirect testimony to this truth is borne by the fact that even at the present time the rationalistic opponents of a supernatural explanation are entirely disagreed as to how the phenomenon is to be accounted for.
What actually takes place may be thus briefly described: in a silver reliquary, which in form and size somewhat suggests a small carriage lamp, two phials are enclosed. The lesser of these contains only traces of blood and need not concern us here. The larger, which is a little flagon-shaped flask four inches in height and about two and a quarter inches in diameter, is normally rather more than half full of a dark and solid mass, absolutely opaque when held up to the light, and showing no displacment when the reliquary is turned upside down. Both flasks seem to be so fixed in the lantern cavity of the reliquary by means of some hard gummy substance that they are hermetically sealed. Moreover, owing to the fact that the dark mass in the flask is protected by two thicknesses of glass it is presumably but little affected by the temperature of the surrounding air. Eighteen times in each year, i.e. (1) on the Saturday before the first Sunday in May and the eight following days, (2) on the feast of St. Januarius (19 Sept.) and during the octave, and (3) on 16 December, a silver bust believed to contain the head of St. Januarius is exposed upon the altar, and the reliquary just described is brought out and held by the officiant in view of the assembly. Prayers are said by the people, begging that the miracle may take place, while a group of poor women, known as the "zie di San Gennaro" (aunts of St. Januarius), make themselves specially conspicuous by the fervour, and sometimes, when the miracle is delayed, by the extravagance, of their supplications.
The officiant usually holds the reliquary by its extremities, without touching the glass, and from time to time turns it upside down to note whether any movement is perceptible in the dark mass enclosed in the phial. After an interval of varying duration, usually not less than two minutes or more than an hour, the mass is gradually seen to detach itself from the sides of the phial, to become liquid and of a more or less ruby tint, and in some instances to froth and bubble up, increasing in volume. The officiant then announces, "Il miracolo é fatto", a Te Deum is sung, and the reliquary containing the liquefied blood is brought to the altar rail that the faithful may venerate it by kissing the containing vessel. Rarely has the liquefaction failed to take place in the expositions of May or September, but in that of 16 December the mass remains solid more frequently than not.
A miracle? Not a miracle? Who knows? (Well, except for Him, of course...)
But an interesting phenomenon nonetheless.