'The chemist's dream' which is used in Freud's 'The interpretation of dreams' to illustrate the use of symbols in dreams, is shown to make use of a mode of symbolic representation drawn from the dreamer's special field of knowledge, namely the science of chemistry. This may well be the earliest example of a device for maintaining the privacy of dream symbols by selecting them from an area which is neither investigated by the analyst (such as the dreamer's personal history) nor one which is part of the analyst's general knowledge (e.g. literature, history, current events, etc.).