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 898 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 effected 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 
always found a loss of weight, but there were differences between one 
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.’’ 
In comparing all these claims, does not the merit of having first 
perceived the 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. Wait, jun.) 
