Solid Lewis adducts comprising MgCl2, a Lewis base belonging to ethers, esters, ketones, silanes or amines and an alcohol ROH, in which R is a C1-C15 hydrocarbon group optionally substituted with heteroatoms containing groups, which compounds are in molar ratios to each other defined by the following formula MgCl2mn in which m ranges from 0. In particular, the elements are joined together by ceramic rods. The housing has therein a liquid pressure chamber to contain oil.