On ihe mperfeEl cf Jloiu tranfttion of Heat through Charcoal, 499 



burned, in the clrcumftances of the prefent experiment, the upper or greateft part of the 

 mafs becomes acidified, and covers a portion beneath, fo as to prevent that lower part from 

 combining with the atmofpheric oxygen ; in confequence of which, tlie combuftion ceafes 

 before the whole is burned. But when this denfe acid becomes diluted with water, it boils, 

 and mod probably rifes, at leaft fo far as to leave the furface of tlie phofphorus below 

 uncovered, and to permit the combuftion to be renewed. Inftances of volatility being given 

 to various fubftances by water, are fufficientiy numerous in chemillry ; of which tlie acid of 

 borax is one of the moft ftriking. To what degree, the fame effect may take place in the 

 phofphoric, I am not aware. Scheele (on Air and Fire, §. 73), indeed, affirms, that water gives 

 it fixity ; but he fays this in fo loofe a way, that I am not dlfpofed to rely on the affertion, as 

 nniverfally true, in contradiftion to fome other fafts, which fhew tliat the diluted acid does 

 partly rife by heat. 



VIII. 



Pyrometr'ical EJfays to determine the Point to which Charseal is a Non-cofiduclor (f Heat. 



By Citizen GuTTON*. 



s. 



'INCE the experiments and refearches of philosophers have been directed to the matter of 

 heat, the flate in which it is found, and the various manners in which it affedts bodies, it 

 has been well afcertained that charcoal is one of die worft conductors of heat. From this- 

 obfervation it has been propofed to form a double wall to furnaces, and to fill the in- 

 termediate fpace -with charcoal ; and, very recently, an happy application of this property has 

 been made in the conftrudtion of veffels defigned for the preferving the temperature of warm 

 infufions. 



I am not, however, acquainted with any refearches which may have been matle to fix,, 

 even comparatively, the limits of this kind of infulation. The effe£t of the non-condu£ting 

 power of charcoal has been fo (lightly confidered, even by the moft accomplifhed chemifts, 

 that they have not tliought it neceffary to take any account of this effect in the refult of their 

 operations. Hence it is, that the celebrated Klaproth, in a fei-ies of experiments upon the 

 aJteration which ftones undergo when expofed to extreme heat, feems to think that the 

 heat might have been nearly equal in fuch as were contained in crucibles of porcelain, and 

 tliofe which were entirely furrounded with charcoal. The redudion of the oxides of 

 tungften, titanium, and uranium, has alfo been attempted in crucibles of charcoal, though it is 

 well known that the higheft degree of heat is here required for tlie fuCon, and that, in other 

 rcfpects, every endeavour muft be made to expofe them to the moft intenfe ignitioai. 



Thefe reflections have led me to conclude, that it might be of fome importance to ebtain 

 a more accurate meafure of this infulating effe£t of charcoal. The following are die experi- 

 ments which I have made upon this fubjedt. 



Out of the fame parcel I took two pyromctic pieces perfecily alike, and pbced one, which 

 Iflliall call A, in a crucible filled with pure filiceous fand dried over the .fiire. The crucible 



• Read to the French National luftitnte, 6th Germinal, in ths 6th year of the Republic (March zS, 1798), 

 and inferwd in the Annales de Chimie, XXVI. 225. 



was 



