46 EEPORT — 1881. 



rotate ; a particular degree of refraction has been imparted ; or rays of 

 certain wave-lengths have been removed by absorption, their absence 

 being manifested by bands in the absorption-spectrum of the substance. 

 The volumes occupied by molecular quantities are dependent partly on the 

 size of the molecules and partly on that of the intermolecular spaces. 



The duty of the physical chemist is to endeavour to co-ordinate his 

 physical observations with the known constitution of compounds as 

 already determined by the pure chemist. This endeavour has in various 

 branches of physical chemistry been to some extent successful. Le Bel 

 has found that among organic compounds those only possess action on 

 the plane of polarised light which contain at least one asymmetric car- 

 bon atom — -that is to say, a carbon atom which is united to four different 

 atoms or groups of atoms. The researches of Landolt, of Gladstone, 

 and of Briihl on the specific refraction of organic liquids, have shown 

 that from the known constitution of a liquid organic compound it is 

 possible to calculate its specific refraction. Noel Hartley, in an examina- 

 tion of the absorption-spectra of organic liquids for the ultra-violet rays, 

 has demonstrated that certain molecular groupings are represented by 

 particular absorption-bands, and this line of inquiry has been extended, 

 with very interesting results, to the ultra-red rays by Abney and Testing. 

 It is obvious that these methods may in ti>rn be employed to determine 

 the unknown constitution of substances. The same holds true of the 

 investigations of Kopp with regard to the molecular volumes of liquids at 

 their boiling-points, in which he has established the remai'kable fact that 

 some elements always possess the same atomic volume in combination, 

 whereas, in the case of certain other elements, the atomic volume varies 

 in a perfectly definite manner with the mode of combination. This in- 

 vestigation has lately been extended with the best results by Thorpe and 

 by Ramsay. Thermo-chemistry, also, which for a long time, at least as 

 regards that portion which relates to the heat of formation of compounds, 

 consisted chiefly of a collection of single equations, each containing three 

 unknown quantities, is beginning to be interpreted by Julius Thomson, 

 whose experimental work in this field is well known. Many other 

 methods of physico-chemical research are being successfully prosecuted 

 at the present day, but it would go beyond the bounds of this summary 

 even to enumerate these. 



The concordant results obtained by these widely differing methods 

 show that those chemists who have devoted themselves, frequently amid 

 the ridicule of their more practical brethren, to ascertaining by purely 

 chemical methods the constitution of compounds, have not laboured in 

 vain. But the future doubtless belongs to physical chemistry. 



In connection with the rectification of the atomic weights it may be 

 mentioned that a so-called natural system of the elements has been intro- 

 duced by MendelejeflF (1869), in which the properties of the elements 

 appear as a periodic function of their atomic weights. By the aid of this 

 system it has been possible to pi'edict the properties and atomic weights 



