618 BUNSEN MEMORIAL LECTURE. 



relates the storv thiit wlieii ho showed this arrangment to the late 

 Emperor Frederick, then Crown Prinec. the Prince remarked: "'For 

 the lirst time in my life, 1 now know the value of a spot of grease."' 

 In the original Bunsen photometer a small flame Inirning in a closed 

 box fixed on a pivot in the center of a long ])oard illuminated the 

 back of the disk, the relative luminous intensity of the two sources of 

 light under examination being ascerttiined by moving them alternately 

 backward and forward on each side of the disk until, in each case, the 

 spot disappeared. This form of the photometer was afterw'ards mod- 

 ified by other observers, not, according to Bunsen, to its advantage, 

 by omitting the small flame and box and simply moving th(^ disk ))a('k- 

 ward and forward between the two lixed sources of light. Recently, 

 Preece has proposed to reintroduce the principle of the original Bun- 

 sen arrangement of ascertaining the ndative luminosity by always 

 exposing the same side of the disk, and therefore eliminating the error 

 arising from its translucency. In one form or other, the Bunsen 

 photometer has, however, for man}' years been in geiu^ral use. luit 

 recently it has been ]iartially replaced by the shadow photometer. 



In this connection mention nHistl)e madeof twoimportant researches 

 of a i)hysical rathei* than of a purely chemical nature, and character- 

 istic of the mani})ulative as well as of the intellectual power of the 

 author. They refer to the ice and the vapor calorimeters. 



As by means of his l)atterv it was possible for Bunsen to pn^pare 

 small (juantities of the rare metals, so ])v help of his ice calorimeter 

 (Pogg. Ann., 1870 (141), 1) he was a))le to ascertain one of their most 

 important physical pi'operties. It was constructed in order to be 

 able to d(>termine exactly the specitic heats of sul)stances which could 

 only be obtained in small quantities, and to which the usual calorimet- 

 ric methods were therefore inapplicable. Thus it ])ecarae of the great- 

 est theoretical importance to ascertain the specitic heat of indium, of 

 cerium, lanthanum, didymimn.and germanium, and other metals which 

 nvv only obtainable in small (|uantities. The principles of construc- 

 tion and mode of action of the ic(^ calorimeter are so w'ell known that 

 a description of the instrument and its use is here superfluous. The}^ 

 were published in 1870. and ])y its means the atomic weight of indium 

 and the fornuihe of its compounds were rectified, while the doubts 

 arising as to the formuUe of the compounds of other metals were 

 eliminated. 



"Thus Bunsen largely contributed to the confirmation and to the 

 acceptance of the system of atomic weights now in use. and thereb}^ 

 to the rational classification of the elements depending on that 

 system.-'^ 



'StanLslao Cannizzaro, Commemorazione del Socio Straniero. R. W. Bunsen, 

 Rendiconti d. K. Acad, dei Liucei, 8 December, 1899. Roma, 1899. 



