In the year 1780, Mr. Watt took out a patent for a method of copying letters and drawings, and the apparatus was manufactured in a partnership with Mr. Bonito!' and Mr. Keir, under the firm of James Watt and Company. This method consists in writing the letter to be copied with an ink partly soluble in water. It is then pressed against a sheet of thin unsized and wetted paper, and the impress ion then taken is read on the other side of the paper from that on which the impression is taken. The apparatus was constructed in two forms, one was a strong rolling press, with large rollers proper for an office, and of a sufficient size to copy plans and drawings, and the other was a compact rolling press and apparatus for copying letters, en closed in a portable writing desk, which folds up into a moderate compass for travelling. When the original is written in strong characters with a sufficiency of ink, three or four successive copies may be taken off upon as many thin sheets of pa per, and they will be very legible; but when a single copy only is taken, the writing upon it ap pears very black, and the original is not at all in jured by the operation.
In the year 178t Mr. Watt contrived a steam drying apparatus for his relation Mr. Macgrigor of Clober near Glasgow, of which we have already given a drawing and description in the article STEAM DRYING.
In the Philosophical Transactions for 1745, Colonel Cook had proposed a method for warming rooms by the steam of boiling water conveyed in pipes along the walls. This method does not seem to have been practically adopted till 1784.5, when Mr. Watt, who probably did not know of Colonel Cook's proposal, put up an apparatus for heating his study by means of steam. This method has since been extensively applied to heat private houses and manufactories, and has been more recently adopted for conservatories and hot-houses.
The discoveries of Dr. Priestley in pneumatic chemistry had excited very general interest, and Mr. Watt, who had taken a particular interest in the discoveries of his friend and neighbour, seems to have devoted considerable attention to the sub ject. The following account of his labours is given by one of his biographers. See Sapp. Encyc. Brit.
" Early in 1783 he was led, by the experiments of his friend and neighbour, Dr. Priestley, to the important conclusion that water is a compound of dephlogisticated and inflammable airs (as they were then called), deprived of their latent or elementary heat, and he was the first to make known this theory.
"This was done in a letter to Dr. Priestley, dated the 28th April 1783, in which he states the Doctor's experiments to have come in aid of some prior no tions of his own, and supports his conclusions by original experiments. That letter Dr. Priestley re
ceived in London, and after showing it to several members of the Royal Society, he delivered it to Sir Joseph Banks, with a request that it might be read at some of the public meetings of the Society; but before that could be complied with, Mr. Watt, having heard of some new experiments made by Dr. Priestley. begged that the reading might he de layed. These new experiments soon afterwards proved to have been delusive, and Mr. Watt sent a revised edition of his letter to Mr. de Luc on the 26th November of the same year, which was not read to the Society until the 29th April 1784, and appears in the Phil. Trans. for that year, under the title of Thoughts on the constituent parts of Water, and of Dephlogisticated dir, with an account of some ex periments upon that subject.
"In the interim, on the 15th January 1784, a paper by Mr. Cavendish had been read, contain ing his Experiments on the Combustion of Dephlo gisticated and Inflammable Sirs, and drawing the same inference as Mr. \Van, with this difference only, that he did not admit elementary heat into his explanation. He refers in it to his knowledge of Mr. Watt's paper, and states his own experiments to have been made in 1781, and mentions Dr. Priestley; but he does not say at what period he formed his conclusions: he only mentions that a friend of his had given some account of his experi ments in the summer of 1783 to Mr. Lavoisier, as well as of the conclusions drawn from them. It is quite certain that Mr. Watt had never heard of them, and Dr. Blagden has stated that he men tioned at Paris the opinions of both the English philosophers, which were not admitted without hesitation, nor until the French chemists had satis fied themselves by experiments of their own." We have copied the preceding statement as that of Mr. Watt's friend; but a regard for the reputa tion of Mr. Cavendish, independent of higher mo tives, compels us to acknowledge that the state ment is partial, and the argument not well founded. We are not able at present to refer to the original documents, but we had occasion some years ago, along with a distinguished chemist, to examine them with minute attention, and it was then our decided conviction, that the merit of the discovery of the composition of water belonged to Mr. Caven dish.
Mr. Watt had the satisfaction of introducing into Great Britain, the new art of bleaching by the oxymuriatic acid, which had been discovered by M. Berthollct; but as the history of his labours has been already given in our article BLEACHING, Vol. III. p. 554, Note, it is unnecessary to resume the subject at present.