54 Inquiry concerning the Nature of Heat, 



of wine in one mass; and if it be too large, the little 

 horizontal column it forms (which I have called a bub- 

 ble) will be ill defined at its two ends, which will ren- 

 der it difficult to ascertain its precise situation. After 

 a number of trials I have found that a tube, the bore 

 of which is of such a size that I inch of it in length 

 contains about 15 or 18 grains Troy of mercury, an- 

 swers best. For a tube of that size the balls may be 

 about i J inch in diameter; and they should both be 

 painted black with Indian ink, which renders the in- 

 strument more sensible. 



I have an instrument of this kind, the tube of which 

 is quite filled with spirit of wine, excepting only the 

 space occupied by a small bubble of air, which is in- 

 troduced into the middle of the horizontal part of the 

 tube; but it does not answer so well as those which 

 contain only a very small quantity of that liquid, suf- 

 ficient to form a small bubble. 



But, without enlarging any farther, at present, on 

 the construction of these instruments, I now proceed to 

 give an account of the experiments for which they were 

 contrived. 



Having found abundant reason to conclude, from the 

 results of the experiments of which an account has 

 already been given, that all the heat which a hot body 

 loses when it is exposed in the air to cool is not given 

 off to the air which comes into contact with it, but that 

 a large proportion of it escapes in rays, which do not 

 heat the transparent air through which they pass, but, 

 like light, generate heat only when and where they are 

 stopped and absorbed, I suspected that in every case 

 when, in the foregoing experiments, the cooling of my 

 instruments was expedited by coverings applied to their 



