CLASSIFICATION. 691 



&c. With the progress of discovery, however, difficulties 

 began to present themselves in such a grouping. Anti- 

 mony, bismuth, and arsenic are distinctly metallic as 

 regards lustre, density, and some chemical properties, but 

 are wanting in malleability. The recently discovered 

 tellurium presents greater difficulties, for it has many of 

 the physical properties of metal, and yet all its chemical 

 properties are analogous to those of sulphur and selenium, 

 which have never been regarded as metals. Great chemical 

 differences again are discovered by degrees between the five 

 metals mentioned ; and the class, if it is to have any che- 

 mical validity, must be made to include other elements, 

 having none of the original properties on which the class 

 was founded. Hydrogen is a transparent colourless gas, 

 and the least dense of all substances ; yet in its chemical 

 analogies it is a metal, as suggested by Faraday l in 1838, 

 and almost proved by Graham ; 2 it must be placed in 

 the same class as silver. In this way it comes to pass that 

 almost every classification which is proposed in the early 

 stages of a science will be found to break down as the 

 deeper similarities of the objects come to be detected. The 

 most obvious points of difference will have to be neglected. 

 Chlorine is a gas, bromine a liquid, and iodine a solid, and 

 at first sight these might have seemed formidable circum- 

 stances to overlook ; but in chemical analogy the substances 

 are closely united. The progress of organic chemistry, 

 again, has yielded wholly new ideas of the similarities of 

 compounds. Who, for instance, would recognise without 

 extensive research a close similarity between glycerine and 

 alcohol, or between fatty substances and ether ? The class 

 of paraffins contains three substances gaseous at ordinary 

 temperatures, several liquids, and some crystalline solids. 

 It required much insight to detect the analogy which exists 

 between such apparently different substances. 



The science of chemistry now depends to a great extent 

 on a correct classification of the elements, as will be learnt 

 by consulting the able article on Classification by Pro- 

 fessor G. C. Foster in Watts' Dictionary of Chemistry. 

 But the present system of chemical classification was not 



1 Life of Faraday, vol. ii. p. 87. 



8 Proceedings of the Royal Society, vol. xviL p. 212. Chemical and 

 Physical Researches, reprint, by Young and Angus Smith, p. 290. 



