512 



Br. D. MendeUeff 



[May 31, 



structural type are like the lifeless pieces on a chess board : they are 

 eudowed but with the voices of living beings, and are not those living 

 beings themselves ; acting, indeed, according to laws, yet each 

 possessed of a store of energy, w^hich, in the present state of our 

 knowledge, must be taken into account. 



In the days of Haiiy, crystals were considered in the same statical 

 and structural light, but modern crystallograjDhers, having become 

 more thoroughly acquainted with their physical properties and their 

 actual formation, have abandoned the earlier views and have made 

 their doctrines dependent on dynamics. 



The immediate object of this lecture is to show that, starting 

 with Xewton's third law of motion, it is possible to preserve to 

 chemistry all the advantages arising fiom structural teaching, with- 

 out being obliged to build up molecules in solid and motionless 

 figures, or to ascribe to atoms definite limited valencies, directions of 

 cohesion, or affinities. The wide extent of the subject obliges me to 

 treat only a small portion of it, namely of suhstitufions, without 

 specially considering combinations and decomj)ositions, and, even 

 then, limiting myself to the simplest examples, which, however, will 

 throw open prospects embracing all the natural complexity of chemi- 

 cal relations. For this reason, if it should prove possible to form 

 groups similar, for example, to H* or CH^ as the remnants of mole- 

 cules CH^ or C-H^ we shall not pause to consider them, because, as 

 far as we know, they fall asunder into two parts, H- -f- H" oi' CH* 

 -f H^, as soon as they are even temporarily formed, and are capable 

 of separate existence, and therefore can take no part in the elemen- 

 tary act of substitution. With respect to the simplest molecules 

 which we shall select — that is to say, those of which the j^arts have 

 no separatp existence, and therefore cannot appear in substitutions — • 

 we shall consider them according to the periodic law, arranging them 

 in direct dependence on the atomic weight of the elements. 



Thus, for example, the molecules of the simplest hydrogen 

 compounds — 



HF 



hydrofluoric acid 



water 



ammonia 



methane 



correspond to elements the atomic weights of which decrease 

 consecutively, 



F = 19, = 16, N - 14, C = 12. 



Neither the arithmetical order (1, 2, 3, 4 atoms of hydrogen) nor 

 the total information we possess respecting the elements will permit 

 us to interpolate into this typical series one more additional element ; 

 and therefore we have here, for hydrogen compounds, a natural base 

 upon which are built up those simple chemical combinations which 

 we take as typical. But even they are competent to unite with each 

 other, as we see, for instance, in the property which hydrofluoric acid 



