WATT. 57 



APPENDIX. 



HISTOEIOAL NOTE OF THE DISCOVERY OF THE THEOEY 

 OF THE COMPOSITION OF WATEE. 



THEEE can be no doubt whatever, that the experiment of 

 Mr. Warltire, related in Dr. Priestley's fifth volume,* gave 

 rise to this inquiry, at least in England ; Mr. Cavendish 

 expressly refers to it, as having set him upon making his 

 experiments. (Phil. Trans. 1784, p. 126.) The experi- 

 ment of Mr. Warltire consisted in firing, by electricity, a 

 mixture of inflammable and common air in a close vessel, 

 and two things were said to be observed : first, a sensible 

 loss of weight ; second, a dewy deposit on the sides of the 

 vessel. 



Mr. Watt, in a note to p. 332 of his paper, Phil. Trans. 

 1784, inadvertently states, that the dewy deposit was first 

 observed by Mr. Cavendish ; but Mr. Cavendish himself, 

 p. 127, expressly states Mr. Warltire to have observed it, 

 and cites Dr. Priestley's fifth volume. 



* Mr. Warltire's letter is dated Birmingham, 18th April, 1781, and 

 was published by Dr. Priestley in the Appendix to the seventh volume of 

 his ' Experiments and Observations relating to various branches of Natu- 

 ral Philosophy ; with a continuation of the Observations on Air,' form- 

 ing, in fact, the fifth volume of his ' Experiments and Observations on 

 different kinds of Air;' printed at Birmingham in 1781. 



Mr. Warltire's first experiments were made in a copper ball or flask, 

 which held three wine pints, the weight 14 ounces ; and his object was to 

 determine " whether heat is heavy or not." After stating his mode of 

 mixing the airs, and of adjusting the balance, he says, he " always accu- 

 rately balanced the flask of common air, then found the difference of weight 

 after the inflammable air was introduced, that he might be certain he had 

 confined the proper proportion of each. The electric spark having passed 

 through them, the flask became hot, and was cooled by exposing it to the 

 common air of the room : it was then hung up again to the balance, and 

 a loss of weight was always found, but not constantly the same ; upon an 

 average it was two grains." 



He goes on to say, " I have fired air in glass vessels since I saw you 

 (Dr. Priestley) venture to do it, and I have observed, as you did, that, 

 though the glass was clean and dry before, yet, after firing the air, it 

 became dewy, and was lined with a sooty substance." 



It seems evident that neither Mr. Warltire nor Dr. Priestley attributed 

 the dew to anything else than a mechanical deposit of the moisture sus- 

 pended in common air. [Note by Mr. James Watt.] 



