28 But if the corn of Titius has become mixed with yours, and this by mutual consent, the whole will belong to you in common, because the separate bodies or grains, which before belonged to one or the other of you in severalty, have by consent on both sides been made your joint property. If, however, the mixture was accidental, or if Titius mixed the two parcels of corn without your consent, they do not belong to you in common, because the separate grains remain distinct, and their substance is unaltered; and in such cases the corn no more becomes common property than does a flock formed by the accidental mixture of Titius's sheep with yours. But if either of you keeps the whole of the mixed corn, the other can bring a real action for the recovery of such part of it as belongs to him, it being part of the province of the judge to determine the quality of the wheat which belonged to each. 29 If a man builds upon his own ground with another's materials, the building is deemed to be his property, for buildings become a part of the ground on which they stand. And yet he who was owner of the materials does not cease to own them, but he cannot bring a real action for their recovery, or sue for their production, by reason of a clause in the Twelve Tables providing that no one shall be compelled to take out of his house materials (tignum), even though they belong to another, which have once been built into it, but that double their value may be recovered by the action called 'de tigno iniuncto.' The term tignum includes every kind of material employed in building, and the object of this provision is to avoid the necessity of having buildings pulled down; but if through some cause or other they should be destroyed, the owner of the materials, unless he has already sued for double value, may bring a real action for recovery, or a personal action for production. 30 On the other hand, if one man builds a house on another's land with his own materials, the house belongs to the owner of the land. In this case, however, the right of the previous owner in the materials is extinguished, because he is deemed to have voluntarily parted with them, though only, of course, if he was aware that the land on which he was building belonged to another man. Consequently, though the house should be destroyed, he cannot claim the materials by real action. Of course, if the builder of the house has possession of the land, and the owner of the latter claims the house by real action, but refuses to pay for the materials and the workmen's wages, he can be defeated by the plea of fraud, provided the builder's possession is in good faith: for if he knew that the land belonged to some one else it may be urged against him that he was to blame for rashly building on land owned to his knowledge by another man.