1835.] Spontaneous Combustion, 121 



but in the latter, especially when it is the object of the 

 manufacturer to make amber rosin, the temperature to 

 which they are exposed is much higher. 



The first experiment I made was on the 16th March. I 

 found the temperature of the boiling rosin, in the still, to 

 be 250° when the turpentine oil and water had been distilled 

 off, the lire just drawn from under the still, and when the 

 liquid rosin was in the act of being strained from the chips 

 which were introduced into the still with the turpentine. 



I had the whole of the chip cake resulting from this 

 distillation carried into my own yard, upon a wire screen, 

 and left in the open air, with a view of watching its progress. 



The temperature increased gradually in the centre of the 

 heap, although externally it became quite cold and brittle. 

 In four hours, in fact, a thermometer thrust into the centre 

 of the porous mass indicated a temperature of 400° ; a good 

 deal of vapour was now given off, and the adhering rosin 

 in the heated parts began to acquire a high colour ; the 

 smell could be perceived at a considerable distance from my 

 premises ; it was a mixed smell of pitch and rosin. 



The chip cake, in this experiment, was first exposed to 

 the air at one o'clock in the afternoon, and, though it rained 

 during the night, at half-past seven the following morning 

 it burst into a flame. 



In a second experiment, I placed the chip cake in an open 

 tar barrel, having three holes bored in its bottom, about 

 two inches diameter each, and it did not take fire till the 

 expiration of thirty-six hours ; but the temperature of the 

 mass was lowered by removal from the wire strainer to the 

 barrel, and besides, I am of opinion the limited access of air 

 retarded the combustion. 



In a third trial which I made, combustion took place in 

 five hours : but in this experiment the temperature of the 

 boiling rosin drawn from the still was 260°, and the chip 

 cake was laid, as in the first experiment, on the wire screen ; 

 the wind, too, was very high. The screen, in this case, was 

 raised a few inches from the ground, in order to let the 

 rosin, as it melted, drip away, which it did in abundance. 



It appeared to me as if the porous mass became slowly 

 red hot, in the centre, like a pyrophorus, and as if the vapour 

 and gaseous matter arising from the decomposed rosin which 



