For nearly half a century, Professor M. A. K. Halliday has been enriching the discipline of linguistics with his keen insights into the social semiotic phenomenon we call language. This ten-volume series presents the seminal works of Professor Halliday. This fifth volume of the collected works explores 'the semantic character of scientific discourse'. It opens with a new essay by Professor Halliday in which he looks at the power of language to make meaning, and addresses the question 'how big is a language'? In the essays that follow Halliday argues that there is no single register of science, but there are numerous scientific discourses. Looking at the history of scientific discourse it is possible to see new strategies evolving which are grounded in processes of metaphor. These grammatical metaphors increase the power that a language has for theorising and hence it is within these metaphors that the power of language resides.