Blue mass was the name of a medicine prescribed, made, and sold in the United States in the 19th century.

Blue mass was recommended as a remedy for such widely varied complaints as tuberculosis, constipation, toothache, parasitic infestations, and the pains of childbirth. It was a magisterial preparation, compounded by pharmacists themselves based on their own recipes or on one of several widespread recipes. It was sold in the form of blue or gray pills, or syrup. Its name probably derives from the use of blue dye or blue chalk (used as a buffer) in some formulations.

The ingredients of blue mass varied, as each pharmacist prepared it himself, but they all included mercury in elemental or compound form (often as mercury chloride, also known as calomel). One recipe of the period included (for blue mass syrup):

Blue mass was also used to treat syphilis, in the form of ointments, gargles, and eye washes.

Mercury is known today to be toxic, and ingestion of mercury leads to mercury poisoning (q.v.), a form of heavy-metal poisoning. While mercury is still used in compound form in some types of medicines and for other purposes, blue mass contained excessive amounts of the metal: a typical daily dose of two or three blue mass pills represented ingestion of more than one hundred times the daily limits set by the Environmental Protection Agency in the United States today.

Blue is a colour, the perception of which is evoked by light having a spectrum dominated by energy with a wavelength of roughly 450–490 nm. It is considered one of the additive primary colours. On the HSV Colour Wheel, the complement of blue is yellow; that is, a colour corresponding to an equal mixture of red and green light. On a colour wheel based on traditional colour theory (RYB) where blue was considered a primary colour, its complementary colour is considered to be orange (based on the Munsell colour wheel).

In Modern English, "blue" is one of the basic colour terms, and one of the seven spectral colours, intermediate between violet (purple) and cyan. It comprises a considerable number of identifiable subcategories that can be identified with descriptive terms like navy blue (a dark blue), cyan blue (or "blue-green", on the boundary to the green range), or sky blue (azure).

The word itself was loaned into Middle English from the Old French word bleu, blo "pale, pallid, discoloured; blue, blue-grey", itself from an Old Frankish *blao.

In physics, mass (from Greek μᾶζα "barley cake, lump (of dough)"), more specifically inertial mass, can be defined as a quantitative measure of an object's resistance to the change of its speed. In addition to this, gravitational mass can be described as a measure of magnitude of the gravitational force which is

when interacting with a second object. The SI unit of mass is the kilogram (kg).

In everyday usage, mass is often referred to as weight, the units of which are often taken to be kilograms (for instance, a person may state that their weight is 75 kg). In scientific use, however, the term weight refers to a different, yet related, property of matter. Weight is the gravitational force acting on a given body—which differs depending on the gravitational pull of the opposing body (e.g. a person's weight on Earth vs on the Moon) — while mass is an intrinsic property of that body that never changes. In other words, an object's weight depends on its environment, while its mass does not. On the surface of the Earth, an object with a mass of 50 kilograms weighs 491 Newtons; on the surface of the Moon, the same object still has a mass of 50 kilograms but weighs only 81.5 Newtons. Restated in mathematical terms, on the surface of the Earth, the weight W of an object is related to its mass m by W = mg, where g is the Earth's gravitational field strength, equal to about 9.81 m/s².