PRINCIPLES OF CHEMISTRY 105 



every compound which possesses the power of turning the plane 

 of polarisation to the right, there is another which, while possess- 

 ing the same composition, rotates equally to the left, was made 

 by Pasteur in 1848 when a very young man. His discovery of 

 the relation between the crystalline forms of the several tartaric 

 acids and their action on polarised light led him to perceive the 

 necessity for some kind of theory to account for the internal 

 structure of the molecules of such compounds. If the atoms 

 composing the molecule in one of such a pair of compounds be 

 conceived as arranged in a particular order, then the atoms in 

 the other must be arranged in the same order but inversely, so 

 that if the atoms could be made visible they would be seen to 

 exhibit the relation of an object to its image in a mirror. Twenty 

 years later the subject again attracted attention, and after the 

 study of the lactic acids by Wislicenus, a theory was put for- 

 ward, by the Dutch chemist Van't Hoff, and the French chemist 

 Le Bel, which furnished the necessary clue, and provided the 

 basis for that large department of the subject which has since 

 developed so remarkably under the name " Stereo-chemistry," 

 or chemistry in space. 



But probably nothing has contributed more to the progress of 

 modern chemistry than the closer study of the relations of 

 chemical to electrical phenomena. The fact of the decom- 

 position of water by the voltaic pile in 1800 was soon followed 

 by the isolation of the metals potassium and sodium by Davy, 

 and later the establishment of the quantitative laws of elec- 

 trolysis by Faraday. Then came all the wonders of spectral 

 analysis, and so the still greater wonders to be revealed by the 

 phenomena connected with the discharge of electricity through 

 attenuated gases were not discovered till a good many years 

 later. 



Another circumstance which has greatly assisted progress in 

 chemical research is the development of many of the improved 

 instruments which are now available for the use of the experi- 

 menter. Some of these will be described later on, but the 

 invention of the Sprengel mercury pump in 1864, by which a 

 high vacuum was for the first time easily available, was certainly 

 one of these. So also is the development of the dynamo by 

 which an electric current is now supplied to every laboratory, 

 and made accessible for so many purposes formerly undreamed 

 of. Thus operations in which the current is made to produce 



