He went through a period of bad health towards the latter years of the 1840s but his health improved again in the next decade and he was able to undertake research with much the same vigour as he had in the 1830s.
Siya ang nagpunta sa pamamagitan ng isang panahon ng masamang kalusugan sa huli taon ng 1840s ngunit ang kanyang kalusugan pinabuting muli sa susunod na dekada at siya ay able sa magsagawa ng pananaliksik sa marami sa parehong lakas bilang niya sa 1830s.