Nicholson, from an initial meeting in 1786, became one of William Godwin's closest friends, a major influence on the conception of his Enquiry concerning Political Justice, and his source during more than a quarter-century for up-to-date knowledge in the sciences. He and his wife were among the most attentive of friends during the fatal illness of Mary Wollstonecraft. On the infant Mary Godwin's nineteenth day, Godwin persuaded Nicholson to write a lengthy prediction of his daughter's character based on her physiognomy, according to the system popularized by Lavater.
Nicholson's most important contribution to chemistry was the discovery of the process of electrolysis of water, using the new advances in electricity developed by Alessandro Volta and others. He discovered that the application of electrical currents to water causes the water to break into its component elements, hydrogen and oxygen -- the first chemical reaction produced by electricity.
In addition to being a practicing chemist, Nicholson was instrumental in propagating knowledge in the field. His introduction to Natural Philosophy (1781) was widely known. He founded The Journal of Natural Philosophy, Chemistry and the Arts in 1797, the first scientific journal not under the aegis of a scientific institution. And in 1809, he published the British Encyclopedia; or, Dictionary of arts and sciences. Comprising an accurate and popular view of the present improved state of human knowledge in six volumes, from which the extracts concerning galvanic experimentation in the first decade of the nineteenth century, a process of which he was among the major exponents, are invaluable. This consolidation of the contemporary scientific scene has long been understood to have been instrumental in making Percy Bysshe Shelley "a Newton among poets." What should be equally clear is that the acquaintance of Mary Shelley with this remarkable man, thus begun in infancy, would have amply provided her with a theoretical understanding of the scientific bases on which her novel purports to rest.