Conversation so far:
What was James Watt's first chemical experiment?
James Watt had already found that an aqueous solution of chlorine could bleach textiles, and had published his findings.
When did James Watt start doing chemical experiments?
In late 1786, James Watt discovered that mixing salt, manganese dioxide, and sulphuric acid could produce chlorine. This would improve the cloth bleaching process.
What was James McGrigor's relationship to James Watt?
James McGrigor was James Watt's father-in-law.
What was the name of the company that James Watt founded with Matthew Boulton in 1775?
The new firm of Boulton and Watt was eventually highly successful and James Watt became a wealthy man.
How long did James Watt work with James McGrigor?
With James McGrigor and James Watt's wife Annie, Watt started scaling up the process, and in March 1788, McGrigor was able to bleach 1500 yards of cloth to his satisfaction.
What did James Watt do after many others began experimenting to improve the process of bleaching?
By 1794 James Watt had been chosen by Thomas Beddoes to manufacture apparatus to produce, clean and store gases for use in the new Pneumatic Institution at Hotwells in Bristol.

Question: What was his most important chemical experiment?
Self-contained rewrite: What was James Watt's most important chemical experiment?