105 



medium appeared to be tinged with the same kind and number of 

 colouring particles as the ordinary medium ; but in other cases, in 

 the same mineral, the extraordinary medium was either tinged with 

 a different number of particles of the same colour, or with a colour- 

 ing matter entirely different from that of the ordinary medium. In 

 some specimens of topaz the colouring matter of the one medium 

 was more easily discharged by heat than that of the other, one of 

 the pencils being yellow and the other pink : hence it is a mistake 

 to suppose that in converting yellow topazes into pink by heat, the 

 former colour is changed into the latter; the fact being, that the 

 yellow is discharged by heat, thus leaving the pink unimpaired. 

 Hence it may be ascertained beforehand whether a topaz will receive 

 a pink colour by heat ; for if that colour exist in one of its images, 

 seen by exposing it to a polarized ray, we may predict the success 

 of the experiment. 



In two specimens of emerald it was found that the colouring mat- 

 ter which tinged the ordinary medium in the one, tinged the extra- 

 ordinary medium in the other, and vice versd. 



Observations sur la Decomposition de VArnidon a la Temperature At' 

 mosphe'rique par V Action de I Air et de VEau. Par Theodore de 

 Saussure, Professeur de Mineralogie dans I'Acad&nie de Geneve, 

 Correspondunt de I'Institut Royal de France, fyc. Communicated by 

 Alexander Marcet, M.D. F.R.S. Read December 17, 1818. 

 [Phil. Trans. 1819, p. 29.] 



After some general observations on the changes which starch un- 

 dergoes during the process of germination, and also when acted on 

 by dilute sulphuric acid, in the manner contrived by M. Kirchoff, 

 the author proceeds to show that starch alone, boiled in water and 

 left to itself, forms, at the end of a certain time, a considerable por- 

 tion of sugar, which is crystallizable, and much resembling that of 

 M. Kirchoff. This change takes place at a temperature between 

 68 and 77 of Fahrenheit, with or without access of air. There is 

 also produced, at the same time, a gum possessed of properties ana- 

 logous to that procured by roasting starch, and a peculiar substance 

 which M. de Saussure calls Amidine. There is also formed a body, 

 insoluble in water and in most acids, but which agrees with starch 

 in forming a blue compound with iodine. 



When the air has free access in these experiments, water is abun- 

 dantly formed, carbonic acid is evolved, and a portion of charcoal is 

 deposited. When the solid contents of this solution are examined, 

 they are found greatly inferior in weight to that of the original starch. 

 The loss is referred principally to the formation of water, and only in 

 small part to the carbon carried off in the form of carbonic acid. 



When air is excluded, no water is produced. A little carbonic 

 acid and nearly pure hydrogen are evolved, and no carbonaceous 

 deposit ensues. Whether the presence or absence of air influences 

 the production of sugar, the author has not been able to determine. 



