Calcium Calcium Calcium is a chemical element with symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to its heavier homologues strontium and barium. It is the fifth most abundant element in Earth's crust and the third most abundant metal, after iron and aluminium. The most common calcium compound on Earth is calcium carbonate, found in limestone and the fossilised remnants of early sea life; gypsum, anhydrite, fluorite, and apatite are also sources of calcium. The
Answering the Question: What is Enlightenment? II of Prussia for creating these preconditions. Kant focused on religious issues, saying that "our rulers" had less interest in telling citizens what to think in regard to artistic and scientific issues. Kant answers the question in the first sentence of the essay: "Enlightenment is man's emergence from his self-incurred immaturity (Unmündigkeit)." He argues that the immaturity is self-inflicted not from a lack of understanding, but from the lack of courage to use one's reason, intellect, and wisdom without the guidance of another. He exclaims that the motto of enlightenment is "Sapere aude"! – Dare to be wise! In the