BUNSEN MEMORIAL LECTURE. (jjc, 



In 1887, when 76 years of age, Bunsen publish(Ml the description of 

 a new vapor ealorhneter (Ann. Phys. (%eni., 1887 (81), 1) upol. wi.i.-l, 

 he had tor some time been engaged. It depends on tlie same prin- 

 ciple as the one previously constructed by Joly (Proc. Rov Soc 

 18.S6 (41), 352). The body whose specific heat has to be determined 

 IS hung by a fine platinum wire to the beam of a ])alance tlien 

 brought into saturated aqueous vapor at 100°, and the amount of water 

 deposited on the body while it is being heated is weighed in the vapor 

 this amount being directly proportional to the specific heat. This 

 method gives very accurate results, and differs in some essential respects 

 from that proposed by Joly. In this way Bunsen determined the 

 specific heat of platinum at difierent temperatures, that of glass, and 

 of water inclosed in glass. This latter he found to be OMm (Joly 

 obtained as a mean result 1.00(32). The originality of this idea, arrived 

 at quite independently from Joly, and the degree of accurac\- with 

 which the whole research is worked out, must indeed be considered as 

 a wonderful achievement of a man close upon SO years of age. 



In addition to the work which Bunsen did alone, lam bound to r(>fer 

 to the long and diflicult series of researches on the measurcincnt of the 

 chemical action of light, in all of Avhich I was associated with him. 

 (Pogg., 1855 (96), 378; Phil. Trans., 1857(147), 355, 3,sl. dol: ls.5il 

 (14!»),879; 1863 (153), 139.) For this reason I feel difficulty in criti- 

 cising it. This difficulty is, however, somewhat remo\ed if foi- this 

 lecture I simply quote the opinion of Richard Meyer as found in his 

 Nachruf of Bunsen, with an extract from Ostwald's Classiker. witii 

 which he closes the notice, and as an illustration of Bunsen's litcrarv 

 stj^le add a feAv sentences of the introduction wiiich he wrote to the 

 fourth part of our photochemical researches. 



"The year 1855 was rendered especially memoral)le. as in that year 

 the first communication appeared of the photochemical investigations 

 which Bunsen carried out together with H. E. Roscoe. These rcsc'arches 

 are considered by Ostwald simply as 'the classical e.xamph' for all 

 further researches in physical chemistry.' 



"The investigation is founded on the discovery by Gay-Liissac and 

 Thenard of the action of light on a mixture of equal volumes of chlo- 

 rine and hydrogen, in which an intense illumination produco an explo- 

 sive combination, while with a less intense one the combination 

 proceeds more slowly. So early as 1843 Draper had made use of this 

 property for the construction of an actinometer, to which he gave the 

 name of tithonometer. This, however, first became a reliable instru- 

 ment in the hands of Bunsen and Roscoe. Equipped with this iristru- 

 ment, they have determined the most important laws of (he chemical 

 action of light after overcoming extraordinary experimentnl dilliculties. 

 Subsequently they replaced this apparatus, in conse«juence of the 

 difficulties attending its manipulation, by the much more convenient 

 chloride of silver actinometer. 



"The first point determined was that the chemically actn«' rays are 



