In the short story Motive v. Opportunity, Philip Garrod is a research chemist and the husband of Grace, the niece of the wealthy Simon Clode. Grace would have inherited a third of her uncle's estate but this was disrupted by the arrival of a spiritualist medium Eurydice Spragg. Worried by the undue influence of the medium over Simon Clode, the solicitor Mr Petherick contacts Philip and suggests that he should bring Simon into contact with some reputed spiritualist circles for an assessemnt. Philip acted quickly and brought in Professor Longman. However the Professor was not definitive enough in his conclusions and Simon continued in his beliefs, eventually wanting to draft a new will to leave the bulk of his estate to Eurydice Spragg.
How this is resolved forms the main plot of the story. Part of the conclusion comes from Philip who met Mr Petherick months after the death of Simon Clode. Although he did not mention it specifically, his chemical talents were probably relevant.