ihejluw Tnirjithn of Heat throvgh Fluids confined by Fibrous Maifa: 29 r 



We have feen by the foregoing expenments, how much the propagation of heat in a liquid 

 IS retarded by dimininiiug its fluidity; and who Icnows but this may continue to be the cale 

 as long as any degree of fluidity remains ? 



As the bodies and branches of trees are not coxxrcd in winter by the fnow which protects 

 their roots from the cold atmofphere, it is evident that extraordinary meafures were ne- 

 ceflary to prevent their being frozen. The bark of all fuch trees as are defigned by nature to 

 fupport great degrees of cold, forms a very warm covering ; but this precaution alone would 

 certainly not have been fufficient for their proteaion. The fap, in all trees which are ca- 

 pable of fupporting a long continuance of froft, grows thick and vifcous on the approach of 

 winter. What more important purpofe could this change anfwer, than that here indicated.' 

 Audit would be more than folly to pretend that it anfwers no ufeful purpofe at all. 



We have feen by the refults of the foregoing experiments, how much the fimple em- 

 barraffment of liquids in their internal motions tends to retard the propagation of heat in 

 them, and confequently its paflage out of them : and when we confider the extreme fmall- 



nefs of the velfels in which the fap moves in vegetables, and particularly in large trees ; 



when we recoUeft that the fubttance of which thefe fmall tubes are formed is one of the belt 

 non-conduaors of heat known *;— and when we advert to the additional embarraflrnents 

 to the paflage of the heat which arife from the increafrd vifcolity of the fap in winter, and 

 to thealmoft impenetrable covering for confining heat which is formed by the bark, we fliall 

 no longer be at a lofs to account for the prefervation of trees during the winter, notwith- 

 flanding the long continuation of the hard frofts to which they are annually expofed. 



On the fame principles we may, I think, account, in a fatisfadory manner, for the preferva- 

 tion of feveral kinds of fruit ; fuch as apples and pears, for iiiftance, which are known to 

 fupport, without freezing, a degree of cold which would foon reduce an equal volume of 

 pure water to a folid mafs of ice. 



At the fame time that the compaa flcin of the fruit efleaually prevents the evaporation 

 of its fluid parts, which, as is well known, could not take place without occafioning a very 

 great lofs of heat, the internal motions of thofe fluids are fo much obftruded by the thin 

 partitions of the innumerable fmall cells in which they are confined, that the communica- 

 tion of their heat to the air ought, according to our hypothefis, to bp extremely flow and 

 difficult. Thefe fruits do, however, freeze at laft, when the cold is very intenfe ; but it 

 mufl be remembered that they are compofed almoft entirely of liquids, and of fuch liquids 

 as do not grow vifcous with cold ; and moreover that they were evidently not defigned to 

 fupport for a long time very fevere frofls. 



Parfnips and carrots, and feveral other kin<!s of roots, fupport cold without freczi ,^ flill 



"* I I.itcly by accident liad occafion to obfcrvc a very flriking proof of the extreme difficulty with which heat 

 paffcs in wood. Being prcfent at the foundry at Munich wlien cannons wcrecafliiig, I obferved that the founder 

 ufed a wooden inftrument for ftirring the melted met.d. h was a piece of oak [daik, green, or unftafoned 

 about ten inches fijuare, and two inches thick, with a long wooden handle, which was fitted into a hole in the 

 middle of it. As this indrument was frequently ufed, aiid fomeiimes remiincd a confideraMc lime in the fur- 

 nace, in which the heat was inotl ihtenfc, I was lurprifed to find that it wis not confuined ; but Iw;is Hill more 

 furprifcd, on examining the part of the pUnk which had been immerfed in the melted metal, to find that the 

 li:at had penetrated it to fo inconfidcrahle a depth, that at the diftance of one-twentieth of an inch below its 

 furfacc the wo(«l did not feeni to have hieii in the leaft .ifTcacd hy it. The colour of the wood remained un- 

 clun^'cd, and it did not appear 10 have left even its rnoiftuie. U. 



longer 



