470 JAMES WATT. 



tained no impurity (literally no particle of soot or of sooty 

 matter). After a great number of trials, Cavendish perceived 

 that if a mixture of common and of inflammable air is ignited, 

 a mixture formed of 1000 measures of the former and 423 of 

 the latter, " about one fifth of the common and nearly the 



ley had not perceived any loss of weight; but I do not find this asser- 

 tion anywhere in the Memoirs of the Birmingham chemist. 



Warltire's earliest experiments on the combustion of gas were made 

 in a copper globe which weighed 398 grammes, and the volume was 

 170 centilitres. The author wished " to decide whether heat is, or is 

 not heavy." 



Warltire first describes the method of mixing the gases, and of ad- 

 justing the scales; he then says, "I always carefully weighed the 

 vessel filled with common air, so that the difference of weight after 

 the addition of the inflammable air enabled me to judge whether the 

 mixture had been efiected in the desired proportions. The passage of 

 the electric spark rendered the globe hot. After it had cooled by ex- 

 posure to the air of the room, I suspended it again on the scales. I 

 alwaj-s found a loss of weight, but there were differences between oae 

 experiment and another. The mean loss was 129 milligrammes." 



Warltire continues as follows: "I have exploded my airs in glass 

 vessels since I saw you recently do so yourself (Priestley), and I have 

 observed, as you did, that however dry and clean the vessel might 

 be before the explosion, it was afterwards covered with dew and a 

 black sooty substance.'" 



la comparing all these claims, does not the merit of having first 

 perceived tlie dew belong to Priestley ? 



In the few remarks that Priestley has added to his correspondent's 

 letter, he confirms the loss of weight, and adds, " Still, I do not think 

 that the bold opinion of the latent heat of bodies entering as a sensible 

 part of their weight, can be admitted without making experiments on a 

 larger scale. If that is confirmed it will be a very remarkable fact, 

 and one that will do infinite credit to Warltire's sagacity." 



And Priestley continues, " We must say also, that at the moment 

 when he (Warltire) saw the dew on the interior surface of the closed 

 glass vessel, he said that it confirmed an opinion which he had long 

 entertained, the opinion that common air parts with its humidity when 

 it is phlogisticated." 



It is evident then that Warltire explained the dew by the simple 

 mechanical precipitation of the hygrometric water contained in cam- 

 mon air. — (Note by Mr. \Vatt,jun.) 



