A controversy a good many years ago agitated the philosophical world, as to the discovery of the Composition of Water—whether the merit was due to Watt or Cavendish. One of Watt's letters, dated May 15th, 1784, seems to compress the matter into a nutshell. Writing to his friend, Mr. Fry of Bristol, Mr. Watt says, that "he has had the honour of having had his ideas pirated;" that Dr. Blagden explained his theory to Lavoisier, at Paris; that M. Lavoisier soon after invented it himself; and that "since that, Mr. Cavendish has read a paper to the Royal Society on the same idea, without making the least mention of me." "The one," he continues, "is a French financier, and the other a member of the illustrious house of Cavendish, worth above 100,000l. (1,000,000l.) and does not spend 1000l. a year. Rich men may do mean actions; may you and I always persevere in our integrity, and despise such doings."

Another important point is, that Watt and Cavendish's papers on the discovery were printed under the sole superintendence of Dr. Blagden, secretary to the Royal Society; that Mr. Watt's paper is printed with the erroneous date of 1784, in place of 1783, and that the separate copies of Mr. Cavendish's papers have the erroneous date of 1783, in place of 1784. The obvious effect of these two errors was to give Cavendish the priority over Watt; whereas, by written testimony, Watt's theory is proved to have been known to Priestley in 1782.

It is Dr. Blagden's conduct in the matter that has disturbed the current of scientific history. "It is his testimony," says an able writer in the North British Review, "not appealed to by Cavendish, but gratuitously offered by himself, that contains the allegation that Cavendish mentioned to him and others his conclusions. It is his testimony, gratuitously sent to Crell, that deprives the French chemists, Lavoisier, Laplace, and Monge, of their due share of honour; and it was by his acts that erroneous dates and claims were propagated throughout Europe. Let us impanel, then, a British jury—not of chemists, for their verdict is given—not of the improvers or manufacturers of steam-engines, for they might be partial—but of the highest functionaries of the law, the members of the peerage—let us lay before them these facts, and then tell them that Blagden received an annuity of 500l. from Cavendish; that, at his death, he left him a legacy of 15,000l.; and we will answer for it, that the testimony of Blagden will be rejected, and the priority of Watt affirmed."