222 



REPORTS ON THE STATE OP SCIENCE. 



results of exploding a mixture containing 15 per cent, of Cambridge 

 coal-gas— first with the vessel polished as highly as possible, and second 

 with the surface blackened over. Precautions were taken to ensure 

 that the mixture in the comparison experiments should be of identical 

 composition. The pressures were recorded in the usual way— some- 

 times with a pencil indicator, and sometimes with an optical indicator — 

 the same instrument being used, however, in each set of comparison 

 experiments. Fig. 4 shows superposed the pressure records obtained 

 by the optical indicator in one such comparison. As in the case of the 

 tin-foil lining there is a difference in the rate of cooling, but the difference 

 is here very much greater — more than twice as great. Further, there 

 is undoubtedly a difference in maximum pressureamounting to between 

 two and three pounds per square inch, which is equivalent to about 

 60° C. in temperature ; or, having regard to the higher volumetric heat 

 in the neighbourhood of 2000° C, to perhaps 5 per cent, in thermal 

 energy. , 



Comparing the two records it will be seen that when the walls are re- 

 flecting the gas takes about li times as long to reach a temperature of 



6, 



I 



-1000 

 2500 

 2000 

 1500 

 1000 

 500 



05 



0-IS 



55 



1500° C. as it does when the walls are blackened. The actual heat 

 given to the walls in the two cases must be the same, so that the mean 

 rate of cooling during this period in the one case is about 11 times as 

 great as it is in the other. This proportion remains fairly constant 

 until the temperature has fallen to about 1000° C, when it shows 

 some tendency to diminish. It was found that the precise state of 

 polish of the silver had a great effect on this result — differences in 

 polish hardly appreciable to the eye causing a substantial change in the 

 rate of cooling. In the diagram shown the surface was polished by 

 means of a motor-driven buffing wheel with rouge, and washed with 

 methylated spirit, and then again polished with a leather. 



A number of experiments have also been made with a recording 

 bolometer of silver strip, which was sometimes polished and sometimes 

 blackened. Simultaneous records were taken of the gas-pressure and 

 of the temperature of the bolometer. Two such records in which the 

 pressure curves are identical are shown superposed in fig. 5. The bolo- 

 meter was mounted on a linoleum backing, and there is considerable 

 loss of heat to this backing, which makes the estimate of the absolute 



