1817.] Experiments on boiling Tar. 113 
this point. Feeling the top of the copper tube, and finding it quite 
cool, I was satisfied that there was much loss of heat from the tube, 
and of course from the included air; and the thermometer could 
only indicate the heat of the air. I obtained permission to bend 
back the wood at the joint, and I plunged the naked bulb into the 
tar. It rose slowly to 220°. It was evident that their mode of 
applying the thermometer was quite ineffectual, and I suggested to 
them an easy way of correcting It, wiz. to keep water in the copper 
tube, which would give the true beat of the tar; and as they wish 
to keep the heat down to 210°, the water ina long covered tube 
would not evaporate very fast; a method which I believe they will 
try. 
Once or twice during the operation, when the scum had been 
taken off of the surface of the liquid tar, I poured some water 
upon it froma phial. It spread, and was lost instantaneously, with- 
out any starting or hissing, as when water falls on hot oil; it lying, 
in this case, on the top of the hot fluid. It may be observed that 
the temperature of the tar in the process of tarring ropes varies, 
owing to the frequent addition of ccd tar to supply the loss of that 
which is imbibed and carried off by the ropes, and by the evapora- 
tion. 
When I saw the thermometer at 220°, and the tar thoroughly 
boiling, I again plunged in my finger, and moved it backwards 
and forwards, making three oscillations of six or eight inches. On 
repeating this motion afterwards, and considering it with my stop- 
watch, | think it occupied between two and three seconds of time. 
The tar certainly felt hotter to the skin than when I had tried it 
before ; and it would not have been pleasant to prolong the time of 
immersion ; but the heat did not rise to a painful degree, and it 
ceased immediately on taking out my finger, although the tar ad- 
hered to the skin just as any liquid of similar viscidity would. I 
suffered no inconvenience except a slight brownish stain, which 
disappeared after the common washing of two days. 
When we endeavour to account for the absence of pain and 
injury to the skin ia the experiment, when a far less degree of heat 
in other liquids would produce serious scalds, it appears from the 
above described facts that the immediate cause is the slowness with 
which the tar communicates its heat. It is evident that it is pos- 
sessed of free heat of temperature superior to that of boiling water; 
for the thermometer indicates 220°; but the thermometer rose very 
slowly from 180° to that point. The heat is greater than the hand 
can bear if continued long in the liquid, but it remains in it for 
some seconds of time before it is felt. The phial of water which I 
suspended in the boiling tar did (L have little doubt) actually boil, 
but I know it remained in for a great length of time before it ac- 
quired its highest temperature. ‘ 
But now what is the cause of this extraordinary slowness of 
communication of heat of temperature from a dense heated liquid 
fluid to bodies immersed init? On this I should be glad to see the 
Vou, IX. N° IL, H 
