her fourth book "molecular and microscopic science" was published in 1869 and had taken her 10 years to write. but she started to have doubts about her choice to devote herself to popularising science, instead of concentrating on mathematics alone. of the book she said: "in writing this book i made a great mistake, and repent it - mathematics are the natural bent of my mind. if i had devoted myself exclusively to that study, i might probably have written something useful, as a new era had begun in that science." regardless, the book was another success. it gave an up-to-date description of the latest discoveries revealed through the microscope and was published in two volumes and three parts. in the first part somerville explained the latest thinking on atoms and molecules, the second part covered plant life and the third part explored animal life. the book included 180 illustrations, which caused her publisher great expense.