140 Intelligence and Miscellaneous Articles. 



that there is in the latex of the fig-tree a powerful digestive fer- 

 ment ; and we hope shortly to be able to state what is the com- 

 position of the residue and what the nature of this new vege- 

 table pepsine principle which is capable of thus digesting albu- 

 minoid substances. — Comptes Rendus de VAcademie des Sciences, 

 5th July, 1880, t. xci. p. 67. 



OBSERVATIONS ON THE VAPOUR-DENSITY OF IODINE. 

 BY M. BERTHELOT. 



In the abstract theory of gases it is assumed that the simple 

 gases receive simultaneously one and the same increment of total 

 energy and one and the same increment of vis viva of translation 

 when they undergo one and the same change of temperature. This 

 conception translates the experiments of physicists on the specific 

 heat of gases (the law of Dulong and Petit), their expansion by heat 

 (Gay-Lussac's law), and their compressibility (Mariotte's law). 



Again, it is concluded from the two latter laws that the density 

 of a gas (that is, the ratio between the weight of a given volume of 

 it and the weight of the same volume of air), taken at the same 

 temperature and the same pressure, is, as a principle, constant. 

 The deviations hitherto observed have been attributed to secondary 

 perturbations. 



These three laws have really been demonstrated for three ele- 

 ments only (oxygen, hydrogen, nitrogen) ; they constitute the only 

 scientific foundation on which the physical determination of mole- 

 cular weights, and consequently the numeration of the atoms, in 

 present theories rests. If for certain elements these laws were to 

 cease to be true, in that case the physical definition of the molecular 

 weights of those elements and that of the number of their atoms 

 would become pure conventions. " 



j\ t ow I have already pointed out that the experiments of MM. 

 Kundt and Warburg on the velocity of sound in mercurial gas 

 were irreconcilable with the whole three fundamental laws above 

 recapitulated (Annates de Chimie et de Physique, 5 e serie, t. ix. 



The experiments of M. V. Meyer on the diminution of the gaseous 

 density of iodine and the halogen elements under constant pressure, 

 but at temperatures very remote from one another, are still more 

 opposed to the received laws. These experiments are moreover 

 confirmed and extended by those which M. Troost has just per- 

 formed on the same body, at a constant temperature but under 

 various low pressures, with the great precision which characterizes 

 him*. 



Thus the variation of the vis viva of translation of the molecules 

 of gaseous iodine, under the influence of very high temperatures or 

 of very low pressures, far surpasses the same variation, observed 



* Vide infra, p. 141. 



