INTRODUCTION 23 



possible to obtain oxyi^-ii from mercury oxide, and that it can -i\- 

 up oxygen to various other substances ; in a word, we desire only to 

 express those transformations of which mercury oxide is capable. Or, 

 more concisely, it may be said that the mmjHtxifion of a compound is 

 the expression of those transformations of which it is capable. It is 

 useful in this sense to make a clear distinction between the conception 

 of an element as a x'//'//v/v homogeneous substance, and as a material, 

 but invisible part of a compound. Mercury oxide does not contain 

 two simple bodies, a gas and a metal, but two elements, mercury and 

 oxygen, which, when free, are a gas and a metal. Xeither mercury as a 

 metal nor oxygen as a gas is contained in mercury oxide ; it only contains 

 the substance of these elements, just as steam only contains the sub- 

 stance of ice, but not ice itself, or as corn contains the substance of the 

 seed but not the seed itself. The existence of an element may be recog 

 nised without knowing it in the uncombined state, but only from an in- 

 vestigation of its combinations, and from the knowledge that it gives, 

 under all possible conditions, substances which are unlike other known 

 combinations of substances. Fluorine is an example of this kind. It 

 was for a long time unknown in a free state, and was, nevertheless, recog- 

 nised as an element because its combinations with other elements were 

 known, and their difference from all other similar compound substances 

 was determined. In order to grasp the difference between the con- 

 ception of the visible form of an element as we know it in the free 

 state, and of the intrinsic element (or * radicle,' as Lavoisier called it) 

 contained in the visible form, it should be remarked that compound 

 substances also combine together forming yet more complex compounds, 

 and that they evolve heat in the process of combination. The original 

 compound may often be extracted from these new compounds by exactly 

 the same methods as elements are extracted from their corresponding 

 combinations. Besides, many elements exist under various visible forms 

 whilst the intrinsic element contained in these various forms is some- 

 thing which is not subject to change. Thus carbon appears as charcoal, 

 graphite, and diamond, but yet the element carbon alone contained in 

 each is one and the same. Carbonic anhydride contains carbon, and 

 not charcoal, or graphite, or the diamond. 



Elements alone, although not all of them, have the peculiar lustre, 

 opacity, malleability, and the great heat and electrical conductivity 

 which are proper to metals and their mutual combinations. But 

 elements are far from all being metals. Those which do not possess 

 the physical properties of metals are called in>n-ntcft(fi< (or metalloids). 

 It is, however, impossible to draw a strict line of demarcation between 

 metals and non-metals, there being many intermediary substances. 



