^xperitnents on Heat enclud by JFri^kn. 1 1* 



If tlie heat, or any confulerable part of it, were produced ia confcquence of a change 

 in the capacity for heat of a part of the metal of the cylinder, as fuch change could only 

 Be fuperficial, the cylinder would by degrees be exhaulted, or the quantities of heat pro- 

 duced, in any given fhort fpace of time, would be found to diminifh gradually in fuccef- 

 five experiments. To find out if this really happened or not, I repeated the laft-men- 

 tioned experiment feveral times with the utmofl care } but I did not difcover the fmalleft 

 fign of exhauftion in the metal, notwithftanding the large quantities of heat a(Shially given 

 off. 



Finding fo much reafon to conclude, that the heat generated in thefe experiments, or 

 excited, as I would rather choofe to exprefs it, was not furniflied at the expence of the 

 latent heat or combined caloric of the metal, I pulhed my enquiries a ftep farther, and en-, 

 deavoured to ^nd out whether the air did or did not Contribute any thing in the genera- 

 tion of it. 



Experiment No. 2, 



AS the bore of the cylinder was cylindrical, and as the iron bar (m), to the end of 

 which the blunt Heel borer was fixed, was fquare, the air had free accefs to the infide of 

 the bore, and even to the bottom of it where the fridion took place by which the heat 

 was excited. 



As neither the metallic chips produced in the ordinary courfe of the operation of bor-- 

 ing brafs cannon, nor the finer fcaly particles produced in the laft-mentioned expsriments 

 by the fridion of the blunt borer, fliowed any figns of calcination, I did not fee how the 

 air could poffibly have been the caufe of the heat that was produced ; but, in an invefti- 

 gation of this kind, I thought that no pains fhould be fparcd to clear away the rubbii'h, 

 and leave the fubjecSt as naked and open to infpecStion as pofTible. 



In order by one decifive experiment to determine whether the air of the atmofphere had 

 any part or not in the generation of the heat, I contrived to repeat the experiment under 

 circumftances in which it was evidently impqffible for it to produce any cffe£l whatever. By 

 means of a pifton exa£lly fitted to the mouth of the bore of the cylinder, through the 

 middle of which pifton the fquare iron bar, to the end of which the blunt fteel borer was 

 fixed, paffed in a fquare hole made perfe£lly air-tight, the accefs of the external air to the 

 infide of the bore of the cylinder was effeftually prevented. In fig. 3. this pifton {/>) is 

 feen in its place : it is likewife fhown in fig. 7. and 8. 



1 did not find, however, by this experiment, that the exclufion of the air diminifhed in 

 the fmalleft degree the quantity of heat excited by the fridion. 



There flill remained one doubt, which, though it appeared to me to be fo flight as 

 hardly to deferve any attention, I was however defirous to remove. The pifton which 

 clofed the mouth of the bore of the cylinder, in order that it might be air-tight, was fitted 

 into it with fo much nicety by means of collars of leather, and preffed againft it with fo 

 much force, that, notwithftanding its being oiled, it occafioned a confiderable degree of 

 fri£lion when the hollow cylinder was turned round its axis. Was not the heat produced, .. 

 or at leaft fome part of it occafioned, by the fridlion of the pifton ? And as the external air 

 had free accefs to the extremity of the bore where it came in contadl with the pifton, is 

 it not poffible that this air might have had fome fliare in the generation of the heat pro- 

 duced ?. 



Experimeitt: 



