816 REPORT— 1901. 



Gases,' p. 4:^' It would, however, be a considerable restriction of investigatiou 

 to follow out only those laws of nature which have a general application and are 

 free from hypothesis ; for mathematical physics has won most of its successes in 

 the opposite way, namely, by starting from an unproved and unprovable, but 

 probable, hypothesis, analytically following out its consequences in every direction, 

 and determining its value by comparison of these conclusions with the result of 

 experiment.' 



The following Papers were read : — 



1. On the Use of the IWejjhone/or investigating the Rhythmic Phenomena 

 in Muscle. By Sir John Burdon Sanderson, Bart., F.R.S. 



1. An Experiment on the ^ Motor ^ Cortex of the Monkey. 

 By Professor C. S. Sherrington, F.K.S. 



3. Arsenical Pigmentation. By Professor J. A. Wanklyn, M.R.CS. 



The publication nf Buusen's splendid researches on 'A New Series of Organic 

 Compounds containing Nitrogen as a Constituent ' was prefaced by a very 

 remarkable pronouncement in 'Poggendorff's Aunalen ' in the year 1837. The 

 curious liquid known as Cadet's fuming liquor, and discovered in 1760, had for 

 many years been mentioned in the then current chemical literature, and in 

 iiccordance with the views then prevalent among chemists was looked upon as a 

 compound of acetic acid with arsenic. Bunsen's researches had completely set 

 aside that view of the constitution of the liquid, and in bringing his results before 

 the chemical world Bunsen announced that the compounds of arsenic resembled 

 the compounds of nitrogen rather than the compounds of the common metals. 

 Carbon, hydrogen, oxygen, and nitrogen had been called the organic elements. 

 Bunsen hinted that arsenic belonged to the organic elements, and maintained 

 that oxide of kakodyl (which exists in Cadet's fuming liquor) and kakodylic 

 acid (which is obtained by oxidising Cadet's fuming liquor) are organic com- 

 pounds in which arsenic has been substituted for nitrogen. 



The utmost diversity prevails among organic compounds containing nitrogen : 

 some are virulently poisonous and others are harmless ; some are colourless and 

 others are dye-stuifs ; and a like diversity is found in the compounds of arsenic. 



On the present occasion I wish to call attention to an organic arsenical com- 

 pound, which is a red pigment discovered by Bunsen about sixty years ago, and 

 named ' Erytrarsin.' According to Bunsen's analysis, its composition is expressed 

 in the formula CjHioA8,;0.5. 



It is described by Bunsen as being very difficult to obtain, being one of the 

 oxidation products of kakodyl ; but the conditions under which it is produced are 

 so little understood that from 100 gi-ammes of oxide of kakodyl the yield of 

 erytrarsin was only half a gramme. Apparently, however, it would seem that 

 traces of it are frequently, and perhaps always, formed during the preparation of 

 kakodyl. 



In a recent preparation of kakodyl in an unusual manner in my laboratory I 

 have obtained it, and if I am not deceived the yield is not quite so small as wlieu 

 kakodyl is produced in the usual way. The solid hydride of arsenic is said to be 

 a pink solid. Arsenical films, as is well known, vary greatly in tints : they may 

 be black or various shades of brown, and even yellow. Under certain circum- 

 stances it would seem that arsenic enters into combination with carbon and forms 

 a black substance. There is also the well-known yellow sulphuret. In fine, 

 arsenic and its compounds afford abundant scope for great variety of coloration 

 in cases of arsenical pigmentation. 



Kakodyl (which is a compound of carbon, hydrogen, and arsenic) is a liquid 



