366 THE PRINCIPLES OF SCIENCE. 



ability. The more recently discovered and rare 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 by degrees discovered between the five metals 

 just mentioned; and the class, if it is to have any chemical 

 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 ana- 

 logies it is a metal, as suggested by Faraday in 1838, 

 and almost proved by the late Professor Graham ; 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 differ- 

 ence 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 circumstances to overlook ; but in 

 chemical analogy the substances are closely united. The 

 progress of organic chemistry, too, has yielded wholly new 

 ideas of the similarities of compounds. Who, for instance, 

 would recognise without extensive research a close simi- 

 larity between glycerine and alcohol, or between fatty sub- 

 stances and ether. The class of paraffins contains three 

 substances gaseous at ordinary temperatures, several 

 liquids, and some crystalline solids. It required much in- 

 sight to detect the perfect affinity 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- 



m ' Life of Faraday,' vol. ii. p. 87. 



