• fO Oft the Solar and Terrejlrial Rays nuhich cccqfion Heat. 



The various refranglbility of the rays of heat from thofc of light was (hewn by thofc 

 experiments with the prifm, which undoubtedly conftitutes a great point of refemblancc. 

 But he points out a very material difference, confiding in the much greater range over 

 which the rays of heat are difperfed when compared with thofe of light. He has exhibited 

 this by delineating a fpe£lrum of light of a certain aflumed length reprefcnted by the line 

 G Q__ (in Fig. 5, Plate III.), divided into feven parts, according to the dimenfions afllgned 

 to the colours by Newton ; and to this he has drawn perpendicular lines, reprefenting the 

 intenfity of illumination pofleffed by the light at each particular part. The length of the 

 maximum perpendicular being arbitrary, he has aflumed it to be |-y of the whole line, 

 namely, L R at the confines of the yellow and green. The lengths of the other ordiiiates 

 to the curve, namely, K M H I N O P, are alfo determined in relative length and pofitioii 

 from his former experiments. 



To find a figure in the fame manner to exprefs the heating power of the refracted prifmatic 

 rays, which may be called the fpe£lrum of heat, he examined the extent of the invifible 

 rays, and found that its line was in proportion to that of the vifible fpeflrum as 2 to 3, or 

 rather, as fome allowance muft be made beyond the laft ordinate, as 2\ to 3. Whence 

 the whole calorific fpeftrum proves to be as 5^- to 3, with regard to the luminous fpe£trum. 

 And if the maximum of heat be exprefled by a line of the fame length as that which denotes 

 tbe maximum of light, and other ordinates be drawn to exprefs the proportions of degrees 

 indicated by the thermometer at the various points of the whole fpe£lrum, another curve 

 BCDEFGHIKLMNOP will be obtained to exprefs the intenfities of heat. From 

 the mere infpcdlion of thefe two figures, thus drawn as lying upon one another, it is feen 

 how very differently the prifm difperfes the heat making rays and thofe which occafion il- 

 lumination. Thefe rays neither agree in their mean refrangibility nor in the fituation of 

 their maxima. At R, where there is moll light, there is but little heat, and at S, where 

 there is mod heat, there is no light at all ! 



Experiment 21. Theftnes of refraElion of the heat making rays are in a'conjiant ratio to the 

 fines of incidence. A prifm was placed half an inch beyond the laft vifible red colour of the 

 folar fpe£lrum from a prifm, and another at the diftance of an inch, while a third was 

 placed befide the fpedlrum as a ftandard. The two firft exhibited change of tem- 

 perature, as in the former trials of this kind, while the laft underwent -no change. 

 Other experiments, with the like event, were tried with prifms of different refradling angles 

 and "of different kinds of glafs, and alfo of water. Hence, as the vifible part of the rays has 

 long been determined to have the conftant ratio here mentioned, and the invifible part 

 always bore the fame relative pofition to thefe, it muft follow that that part alfo is deflefted 

 according to the fame law. 



Experiment 22. The different refrangibility of heat is correBed by contrary refraElion in 

 different mediums. A prifm of crown glafs, having an angle of 25 degrees, another of 

 flint glafs, with an- angle of 24, and a third of crown glafs, with an angle of 10 degrees, 



being 



