June 27, 1889] 



NATURE 



195 



p .V = const.— z>. Boyle's law — was true for ideal gases 

 only. Between one atmosphere and thirty atmospheres the 

 deviations were positive in the case of hydrogen, and nega- 

 tive in those of all other gases. Mendeleeff pointed out 

 that the deviations must become positive for all gases at 

 sufficiently high pressures, and the fact has since been 

 confirmed by the observations of Amagat and Cailletet. 

 Mendeleeff, more particularly, made observations at low 

 pressures, i.e. below one atmosphere ; and here the de- 

 viations were again found to be positive and relatively 

 very large. It was found, in fact, that, at the limit of 

 •condensation, the gases seemed to behave like solid 

 bodies— z>. the molecules were incapable of being 

 stretched or brought nearer together to any appreciable 

 extent by varying pressure. Mendeleeff has further de- 

 termined the real coefficients of thermal expansion of 

 gases. This, for air between 0° and 100° under a standard 

 atmosphere, was found to be o 0036829. Determinations 

 made in the case of other gases have shown that the co- 

 efficients of expansion increase with increasing molecular 

 weight, gases of the same molecular weight giving the 

 same coefficient. 



Molecular weight. Coefficient of expansion. 



Hydrogen 2 ... 000367 



Nitrogen \ ^g o-oo^7^ 



Carbon monoxide ... / "■ •"•' 



Nitrous oxide \^ n-ooiTi 



Carbon dioxide ...J ^ "' ^'^ 



Sulphur dioxide ... 64 ... o "00385 



Hydrogen bromide... 81 ... 0'C0386 



The coefficient of expansion is found to decrease with 

 increasing pressure in the case of hydrogen. Thus at 



200 mm 0'00369 



760 ,, o 00367 



8 atmos o'oo366 



But with the so-called coercible gases the reverse is 



found to take place. Thus, in the case of carbon 

 dioxide, 



120 mm. pressure 0*00372 



220 ,, ,, 000370 



760 ,, „ 0-00373 



3 atmos. ,, 0-00389 



^ ,, „ OG0413 



The decrease of the coefficient of expansion with in- 

 creasing pressure is a normal phenomenon of gases, the 

 positive deviation observed in the case of hydrogen being 

 found to hold equally good for all gases at very high and 

 very low pressures. Hence the laws of Boyle and Charles 

 are only valid at points of the curve when the deviation 

 changes from positive to negative or vice versa. 



These experiments have also borne fruit in various 

 meteorological papers on the physical nature of the highly 

 rarefied air existing in the upper strata of the atmosphere. 

 In this connection it may be stated that Mendeleeff has 

 attempted to organize meteorological observations in the 

 upper regions of the atmosphere by means of balloons, 

 and hence he has been led to study aeronautics. His 

 practical acquaintance with the subject induced him to 

 make an ascent from Klin during the total solar eclipse of 

 August 19, 1887, for the purpose of observing the exten- 

 sion and structure of the corona when seen through highly 

 rarefied air. 



Russia is indebted to Mendeleeff for the training of 

 two generations of her chemists. His writings have 

 largely modified the mode of teaching chemical science 



in that country. His treatise on Organic Chemistry was 

 the standard work of its time, and exercised great in- 

 fluence in spreading abroad the conceptions which are 

 associated with the development of modern chemistry. 

 His " Principles of Chemistry," published in 1869, and 

 repeatedly reprinted, is a veritable treasure-house of 

 ideas, from which investigators have constantly borrowed 

 suggestions of new lines of research. This book is one 

 of the classics of chemistry ; its place in the history of 

 science is as well assured as the ever-memorable work of 

 Dalton. Mendeleeff, indeed, might with equal fitness 

 have styled his book a " New System of Chemical Philo- 

 sophy." In it he has developed the great generalization 

 which is known under the name of the " Periodic Law" — 

 a generahzation which is exerting a profound influence on 

 the development of chemical science in all countries in 

 which its study is actively prosecuted. Mendeleeff first 

 drew attention to the principles upon which the Periodic 

 Law is based in a paper read to the Russian Chemical 

 Society in 1869, in the following series of propositions : — 

 (i) The elements, if arranged according to their atomic 

 weights, exhibit an evident periodicity of properties. 



(2) Elements which are similar as regards their chemi- 

 cal properties have atomic weights which are either of 

 nearly the same value {e.g. platinum, iridium, osmium), 

 or which increase regularly {e.g. potassium, rubidium, 

 caesium). 



(3) The arrangement of the elements, or of groups of 

 elements in the order of their atomic weights, corresponds 

 to their so-called valencies., as well as, to some extent, to 

 their distinctive chemical properties ; as is apparent 

 among other series in that of lithium, beryllium, barium, 

 carbon, nitrogen, oxygen, and iron. 



(4) The elements which are the most widely diffused 

 have small atomic weights. 



(5) The magnitude of the atomic weight determines the 

 character of the element, just as the magnitude of the 

 molecule determines the character of a compound body. 



(6) We must expect the discovery of many yet unknown 

 elements — for example, elements analogous to aluminium 

 and silicon— whose atomic weight would be between 65 

 and 75. 



(7) The atomic weight of an element may sometimes be 

 amended by a knowledge of those of its contiguous ele- 

 ments. Thus the atomic weight of tellurium must lie 

 between 123 and 126, and cannot be 128. 



(8) Certain characteristic properties of elements can be 

 foretold from their atomic weights. 



In the Faraday Lecture recently delivered to the 

 Chemical Society, and from which these words are taken, 

 Mendeleeff has indicated for us the lines upon which the 

 evolution of his theory proceeded. In the first place, it 

 is to be noted that it is based wholly on experiment : it is 

 as much the embodiment of fact as are the laws of 

 chemical combination formulated by Dalton. Without 

 the knowledge of certain data it could not possibly 

 have been discovered ; with this knowledge its appearance, 

 says Mendeleeff, is natural and intelligible. Three 

 series of data were necessary to pave the way for its 

 enunciation : — 



(i) The adoption of the definite numerical values of 

 the atomic weights founded on the conceptions of 

 Avogadro and Gerhardt, as insisted upon by Cannizzaro. 



