1900.] on Bunsen. 443 



by electricity, and made the first step towards the modern system of 

 arc lighting. In 1852 Bunsen turned his attention to ai)plying his 

 battery to the electrolytic production of metals, the reduction of 

 which had hitherto baffled the attacks of the chemist. Magnesium 

 first yielded, next came the metals of the alkaline earths — calcium, 

 strontium, and later on cerium, lanthanum and didymium. In 1856 

 Bunsen and I determined the actinic value of the light of burning 

 magnesium ; we found that this was so great that it might be used for 

 photographic purposes, and the first photographic portrait by the 

 magnesium light was taken by myself in this room on May 6, 1864, 

 when, during a lecture which I gave, Faraday was photographed as he 

 sat, and also Sir Henry Holland, who presided on that occasion. 



Another well-known instrument invented by Bunsen (1844) is 

 his photometer, used now almost exclusively for measuring the 

 illuminating power of coal gas. The essential feature of this appa- 

 ratus is a disc of paper having a grease spot in the centre, a 

 comparison of the luminous intensity being made when by the ap- 

 proximation of the source of light to the illuminated disc the 

 spot becomes invisible. When this instrument was shown and 

 explained to the late Emperor Frederick he remarked, " For the first 

 time in my life I now know the value of a spot of grease." 



The only relaxation from his scientific labours which Bunsen 

 throughout life allowed himself was travelling, and this he thoroughly 

 enjoyed. During many autumn vacations, I had the pleasure of 

 accompanying him in rambles throughout Switzerland and the Tyrol. 

 He walked well, and had a keen appreciation of natural beauty, 

 especially of mountain and woodland scenery, whilst he took great 

 interest in the geology and physical characteristics of the districts 

 through which he passed, and this it was that led him to turn his 

 mind to chemico-geological studies. So early as 1844, in company 

 with Pilla and Matteucci, he visited and carefully examined the car- 

 boniferous deposits occurring in the well known fumerole districts 

 of the Tuscan Maremma, and in 1846 he undertook his journey to 

 Iceland, where he spent three and a half months, and the outcome of 

 which was the well-known series of investigations on the volcanic 

 phenomena of that island. No doubt it was the eruption of Hecla in 

 1845 which served as the incentive to this expedition, for he desired 

 not only to examine the composition of the Icelandic rocks, which 

 are entirely of volcanic origin, but especially the pseudovolcanic 

 phenomena which present themselves in greater force immediately 

 after a period of activity than at other times. 



The expedition to Iceland was an official one promoted by the 

 Danish Government. Bunsen was accompanied by Sartorius von 

 Waltershausen and Bergman, both colleagues at Marburg, as well as 

 by the French mineralogist Des Cloizeaux. They left Copenhagen 

 on May 4, 1846, reaching Keykiavik after a short but stormy passage 

 of eleven days. The party spent ten days at the foot of Hecla, where 

 Bunsen collected the gases emitted by the fumeroles, and investigated 



2 g 2 



