PHILOSOPHICAL TRANSACTIONS. 



I. On Calorescence. By Professor J. Tyndall, LL.B. Camb., F.B.S., Member of the Acade- 

 mies and Societies of Holland, Geneva, Gottingen, Zurich, Halle, Marburg, Breslau, 

 Upsala, Cherbourg, la Societe Philomathique of Paris, Cam. Phil. Soc. &c.; Professor 

 of Natural PMlos(yphy in the Boyal Institution and the Boyal School of Mines. 



Received October 20,— Eead November 23, 1865. 



Forsitan et rosea sol alte lampade lucens 

 Possideat miiltum eaeeis fervoribus ignem 

 Circum se, nullo qui sit fulgore notatus, 

 .i^Estiferum ut tantum radiorum exaugeat ictum. 



Ltjcbet. v. 610*. 



In the year 1800, and in the same volume of the Philosophical Transactions that con- 

 tains Volta's celebrated letter to Sir Joseph Banks on the Electricity of Contact f, Sir 

 William Herschel published his discovery of the invisible rays of the sun. Causing 

 thermometers to pass through the various colours of the solar spectrum, he determined 

 their heating-power, and found that this power, so far from ending at the red extremity 

 of the spectrum, rose to a maximum at some distance beyond the red. The experiment 

 proved that, besides its luminous rays, the sun emitted others of low refrangibility, 

 which possessed great calorific power, but were incompetent to excite vision. 



Drawing a datum-line to represent the length of the spectrum, and erecting at various 

 points of this line perpendiculars to represent the calorific intensity existing at those 

 points, on uniting the ends of the perpendiculars Sir William Herschel obtained the 

 subjoined curve (fig. 1), which shows the distribution of heat in the solar spectrum, 

 according to his observations. The space A B D represents the invisible, and B D E the 

 visible radiation of the sun. With the more perfect apparatus subsequently devised, 

 Professor Muller of Freiburg examined the distribution of heat in the spectrum J, 



* I am indebted to my excellent friend Sir Edmund Head for this extract, which reads like divination, 

 t Vol. Ixx. + Philosophical Magazine, Ser. 4. vol. xvii. p. 242. 



MDCCCLXVI. . B 



