VOL. LXVII.] PHILOSOPHICAL TRANSACTIONS. - 203 



We do not at all know what this difference could be owing to, especially in 

 the observations in steam. It could not proceed entirely from some unknown 

 difference in the water ; for if it did, the difference between the different ther- 

 mometers should have been always the same, which was not the case, though in 

 general, on those days in which one thermometer stood high, the others did 

 also, especially in the trials in steam. Also, as far as can be perceived from our 

 experiments, there seems to be very little difference between different waters, 

 with respect to the heat which they acquire in boiling. We could not be sure 

 that there was any difference between rain or distilled water and pump-water, 

 provided the latter had boiled long : neither did any difference seem to arise from 

 the water containing such substances as are disposed to part readily with their 

 phlogiston ; for, on trying the thermometers in the steam of distilled water, 

 their height was not sensibly altered by pouring in a small quantity of a solution 

 of liver of sulphur, or of iron filings imperfectly rusted. The thermometer 

 however seemed to stand sensibly lower in pump water beginning to boil, than 

 in the same water long boiled, but the difference scarcely exceeded -pL or J- of a 

 degree. 



We made some experiments to determine the heat of water boiling in open 

 vessels. In general, when the vessel was almost full, and the water boiled fast, 

 and the ball of the thermometer was held from 4 to 2 or 3 inches under water, 

 and also in that part of the vessel where the current of water ascended upwards, 

 that is, in the hottest part of the water, its heat was not much different from 

 that of the steam of water boiling in closed vessels, varying only from a quarter 

 of a degree more than that, to as much less ; but if the water boiled gently, its 

 heat would frequently be half or three-quarters of a degree cooler than the steam. 

 If the experiment was tried in the deep pot with such a quantity of water in it 

 that the surface was at least 1 4 or 15 inches below the top of the pot, so that 

 though the vessel was open, yet the water was not much exposed to the air, its 

 heat then seemed scarcely less than when boiled in closed vessels. 



In making these experiments, we chiefly made use of the 2 short thermo- 

 meters, in which, as the quantity of quicksilver contained in the tube was 

 small, the error arising from that part of the quicksilver being not heated equally 

 with that in the ball, could be but small : for example, in the 2d of the short 

 thermometers, the number of degrees contained in that part of the tube between 

 the circular plate Gg and the ball was 18". In the experiments in steam, this 

 part of the tube was heated to the same degree as the ball. Suppose now, that 

 in open vessels it was heated only to 112", or was 90° cooler than the ball, it is 



18 X *^0 



plain that the thermometer would stand at only ' , or j- of a degree lower 



than it did in steam, provided the heat of the quicksilver in the ball was the 



