2H THE POPULAR SCIENCE MONTHLY. 



teach us the hahits 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 measura- 

 ble changes, the nature and amount of which are assuredly condi- 

 tioned by the mass, form, and other properties of the molecules : the 

 plane of polarization has been caused to 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 mole- 

 cules and partly on that of the intermolecular spaces. 



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

 physical observations with the known constitution of compounds as 

 already determined by the pure chemist. This endeavor has in va- 

 rious 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 polarized light which contain at least one asym- 

 metric 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 Brtihl on the specific refraction of organic liquids, 

 have shown that from the known constitution of a liquid organic com- 

 pound 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 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 Festing. It is obvious that these methods may in turn 

 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 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 man- 

 ner 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 formation of compounds, con- 

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

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

 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 sum- 

 mary 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 



