48 WATT. 



steam-engine is the only monument of his scientific 

 genius or his inventive skill. He was the author of 

 the machine in general use for copying letters ; of the 

 method extensively used for heating buildings and hot- 

 houses by steam ; and of an ingenious mechanism for 

 multiplying copies of busts and other sculptures ; but 

 he was also, without any doubt, the person who first 

 discovered the composition of water. At this most 

 important truth he arrived by a profound examination 

 of all the experiments which had been made by Warl- 

 tire, by Macquer, and especially by Priestley, upon 

 the combustion of hydrogen and oxygen gases, then 

 called inflammable, and vital or dephlogisticated airs 

 No former reasoner had come even near the true theory 

 of the phenomena observed in those experiments. All 

 had assumed that water was a simple or elementary 

 body ; that it was contained in the airs burnt together, 

 and was precipitated by their explosion. He, on the 

 contrary, showed that it was formed by the union of 

 the two gases, and their parting with the latent heat 

 which had held them in the elastic or gaseous state, 

 but which being withdrawn by their union, left them 

 in a state of liquid or aqueous fluidity. 



As early as 1782, his attention had been closely 

 directed to the experiments in which air is produced 

 from water, and especially to those upon the com- 

 bustion of inflammable air. In December of that year 

 he had matured his theory, for we find him then an- 

 nouncing to De Luc his discovery, that " one element 

 must be dismissed from the list ;" water being, accord- 

 ing to his doctrine (stated more explicitly to Dr. Black, 

 April 1783,) " composed of dephlogisticated and in- 

 flammable airs deprived of a portion of their latent 

 heat." To his whole correspondence with that great 

 philosopher, with Smeaton, with Priestley, De Luc, and 

 others, I have had access, and no trace is to be found 

 in it that either he or they had even entertained the 

 least suspicion of the same thing having before oc- 



