Mr. J. Davies on the Spontaneous Combustion of Charcoal. 89 



brana versicolor on the internal surface when wet. As soon 

 as the whole is thoroughly dry, examine it again, and you will 

 find that all the colours have vanished. Immerse the collapsed 

 membrane in water (or for more expedition in tepid water), 

 and as the parts are gradually expanded you will find that the 

 colours will be restored as beautiful as ever. This you may 

 repeat as often as you like, with the same result. 



In the choroid of a sheep, which I had injected minutely 

 with vermilion, the effect of this experiment was most beau- 

 tiful. The original colour of the membrana versicolor was a 

 rich blue, but when dry the red injection was alone visible. 

 With a little management I contrived to get it under the field 

 of my microscope so as to see the actual process of the change. 

 The blue surface came stealing over the red injection like a 

 cloud, and gradually increased in density till the injection was 

 fairly obscured by it. 



XVI. An Attempt to assign the Cause of the Spontaneous Com- 

 bustion of Charcoal. By Mr. John Davies, Member of the 

 Werner ian Society of Edinburgh, Sfc. and Lecturer on Che- 

 mistry, $c* 



N the interesting paper which has just been readf, no at- 

 tempt has been made by the author of it to furnish an ex- 

 planation of the phaenomenon which he has established and 

 described : and as every inquiring mind will direct its atten- 

 tion to a rationale of the operation, I presume that some re- 

 marks which have occurred to me within the last few days, 

 and which may at least supply some aid in the discussion, may 

 be, without impropriety, appended to the paper. These re- 

 marks, though theoretical, are countenanced by experimental 

 analogies, which, if they fail to establish the accuracy of the 

 speculation, may at least excuse its introduction. 



A statement of the mode in which the charcoal in question 

 is made, will be necessary in the subsequent explanation. 

 Small fragments of wood, generally stripped of their bark, are 

 put into iron cylinders, and exposed to intense heat, in order 

 to effect the distillation of the volatile constituents for the ma- 

 nufacture of iron liquor. Now Mr. Brunner resorted to a 

 similar procedure in obtaining potassium from potash and 

 charcoal; and, as we know that potash may be procured 



I 



* Read before the Literary and Philosophical Society of Manchester, 

 November 4, 1831 ; and now communicated by the Author. 



f Mr. Davies here alludes to Mr. Hadfield's paper on the same subject, 

 given in our last Number, p. 1. — Edit. 



Third Series. Vol. 3. No. 14. Aug. 1833. N 



