The C-14 dating technique measures time by radioactive decomposition of materials. There are other clocks which depend on the chemical decomposition of materials. The forgery of the Piltdown Man fossil was detected by a dating method which depends on the decomposition of bone protein and its replacement by fluorine. Fluorine is an element which occurs naturally as a gas, but which combines readily with other elements to form compounds. Some of these elements are common in bones. Since fluorine is one of nature’s most reactive elements, it tends to escape from the compound it is in and to form other compounds. As the protein in a bone decays, it is often replaced by fluorine. The amount of fluorine in old bones, then, is expected to be more than in young bones since it has had more time to accumulate. Since fluorine does not accumulate at a constant rate, it affords only a relative measure of age. In the case of Piltdown, a group of bones was discovered at a site, and was said to contain those of one individual who lived about 60,000 years ago. Almost 100 years after they were discovered, these bones were put to the fluorine test. It was found that (1) some of the bones had less fluorine than others, so not all were of the same antiquity and could not have belonged to the same individual, (2) the younger bones had as much fluorine in them as modern bones, and (3) the older bones had more fluorine in them than bones known to be 60,000 years old. [Illustration: (uncaptioned)] Another form of absolute dating of importance to archaeology is that called the _varve_ method. This can only be utilized as an absolute clock under very special circumstances however. Varves are like tree rings in a way. In a lake which is sufficiently deep, or at the edge of a glacier, particles of sediment are being deposited continuously as a sort of fallout from the water. During the winter, when the glacier freezes or the density of the water in the lake increases because of the cooler temperature, less particles are deposited, and those which are deposited are usually of a characteristic color or texture. During the summer, when the glacier melts or the lake warms up, more and different particles are deposited. The bands of deposited sediment are called varves; every year two varves are formed. Starting from the top, one can count back the number of years in a varve series. If the top varve is of known date such as the present year, one has a calendar with each varve having a known date. Attempts are made to correlate one varve series with another in order to recover even longer series. If the archaeologist is lucky, and it is not rare in Europe, there will be materials from a site buried in the local varve sequence. Counting back gives an absolute age for the artifacts embedded in the site and thus an approximate age for the site. The European varve chronology is believed to extend back to about 9650 B.C.