80 ADDRESS TO THE BRITISH ASSOCIATION, 1881. 



on its velocity of transpiration in the liquid or gaseous state these 

 teach us the habits of the living substance. The rays of light which 

 have threaded their way between the molecules of a body have under- 

 gone, in contact with these molecules, various specific and measurable 

 changes, the nature and amount of which are assuredly conditioned by 

 the mass, form, and other properties of the molecules : the plane of 

 polarisation has been caused to rotate ; a particular degree of refrac- 

 tion 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 mole- 

 cular 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 carbon 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 examination of the absorption spectra of. organic 

 liquids for the ultra-violet rays, has demonstrated that certain mole- 

 cular 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 Festing. It is obvious that these 

 methods may in turn be employed to determine the unknown consti- 

 tution 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 remarkable 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 investigation 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 forma- 

 tion of compounds, consisted chiefly of a collection of single equations, 

 each containing three unknown quantities, is beginning to be inter- 

 preted by Julius Thomsen, whose experienced work in this field is well 



