"The representations of friends whose opinions I highly value induce me to avail myself of this opportunity of noticing an error into which not only Dr. Robison, but apparently also Dr. Black, has fallen, in relation to the origin of my improvements upon the steam engine, and which not having been publicly controverted by me, has, I am informed, been adopted by almost every subsequent writer upon the subject of latent heat.
"Dr. Robison, in the article Steam Engine, after passing an encomium upon me, dictated by the partiality of friendship, qualifies me as the 'pupil and intimate friend of Dr. Black,'—a description which not being there accompanied with any inference, did not particularly strike me at the time of its first perusal. He afterwards, in the dedication to me of his edition of Dr. Black's lectures upon chemistry, goes the length of supposing me to have professed to owe my improvements upon the steam engine to the instructions and information I had received from that gentleman, which certainly was a misapprehension; as, though I have always felt and acknowledged my obligations to him for the information I had received from his conversation, and particularly for the knowledge of the doctrine of latent heat, I never did nor could consider my improvements as originating in those communications. He is also mistaken in his assertion (p. 8. of the preface to the above work), that 'I had attended two courses [Pg094] of the doctor's lectures;' for, unfortunately for me, the necessary avocations of my business prevented me from attending his or any other lectures at college; and as Dr. Robison was himself absent from Scotland for four years at the period referred to, he must have been misled by erroneous information. In p. 184. of the lectures, Dr. Black says, 'I have the pleasure of thinking that the knowledge we have acquired concerning the nature of elastic vapours, in consequence of my fortunate observation of what happens in its formation and condensation, has contributed in no inconsiderable degree to the public good by suggesting to my friend Mr. Watt of Birmingham, then of Glasgow, his improvement on this useful engine' (meaning the steam engine of which he is then speaking). There can be no doubt from what follows in his description of the engine, and from the very honourable mention which he has made of me in various parts of his lectures, that he did not mean to lessen any merit that might attach to me as an inventor; but, on the contrary, he was always disposed to give me fully as much praise as I deserved.
"And were that otherwise doubtful, it would, I think, be evident from the following quotation from a letter of his to me, dated 13th February 1783, where, speaking of an intended publication by a friend of mine, on subjects connected with the history of steam, he says, 'I think it is very proper for you to give him a short account of your discoveries and speculations; and particularly to assert clearly and fully your sole right to the honour of the improvements of the steam engine.' And in a written testimonial which he very kindly gave me, on the occasion of a trial at law against a piracy of my invention in 1796-7, after giving a short account of the invention, he adds, 'Mr. Watt was the sole inventor of the capital improvement and contrivance above mentioned.'
"Under this conviction of his candour and friendship, it is very painful to me to controvert any assertion or opinion of my revered friend; yet, in the present case I find it necessary to say, that he appears to me to have fallen into an error; and I hope, in addition to my assertion, to make that appear by the short history I have given of my invention, in my [Pg095] notes upon Dr. Robison's essay, as well as by the following account of the state of my knowledge previous to my receiving any explanation of the doctrine of latent heat; and also from that of the facts which principally guided me in the invention.
"It was known very long before my time, that steam was condensed by coming into contact with cold bodies, and that it communicated heat to them; witness the common still, &c. &c.
"It was known, by some experiments of Dr. Cullen and others, that water and other liquids boiled in vacuo at very low heats; water below 100°.
"It was known to some philosophers that the capacity or equilibrium of heat, as we then called it, was much smaller in mercury and tin than in water.
"It was also known that evaporation caused the cooling of the evaporating liquid, and bodies in contact with it.
"I had myself made experiments to determine the following facts:—
"First, the capacities of heat for iron, copper, and some sorts of wood, comparatively with water.