By the mid-18th century, smallpox was a major endemic disease everywhere in the world except in Australia and small islands untouched by outside exploration. In 18th century Europe, smallpox was a leading cause of death, killing an estimated 400,000 Europeans each year. Up to 10 percent of Swedish infants died of smallpox each year, and the death rate of infants in Russia might have been even higher. The widespread use of variolation in a few countries, notably Great Britain, its North American colonies, and China, somewhat reduced the impact of smallpox among the wealthy classes during the latter part of the 18th century, but a real reduction in its incidence did not occur until vaccination became a common practice toward the end of the 19th century. Improved vaccines and the practice of re-vaccination led to a substantial reduction in cases in Europe and North America, but smallpox remained almost unchecked everywhere else in the world. By the mid-20th century, variola minor occurred along with variola major, in varying proportions, in many parts of Africa. Patients with variola minor experience only a mild systemic illness, are often ambulant throughout the course of the disease, and are therefore able to more easily spread disease. Infection with v. minor induces immunity against the more deadly variola major form. Thus, as v. minor spread all over the US, into Canada, the South American countries, and Great Britain, it became the dominant form of smallpox, further reducing mortality rates.
This equation is known as Grahams law of effusion. The effusion rate for a gas depends directly on the average velocity of its particles. Thus, the faster the gas particles are moving, the more likely they are to pass through the effusion orifice. Knudsen effusion cell
The Knudsen effusion cell is used to measure the vapor pressures of a solid with very low vapor pressure. Such a solid forms a vapor at low pressure by sublimation. The vapor slowly effuses through a pinhole, and the loss of mass is proportional to the vapor pressure and can be used to determine this pressure. The heat of sublimation can also be determined by measuring the vapor pressure as a function of temperature, using the Clausius–Clapeyron relation. == References ==