DISCOVERY 



75 



more than one variety of the solid form of a substance, 

 and cases are also known in which varieties occur in 

 the liquid and gaseous states. 



Newton observed the high specific gravity and the 

 power of refracting rays of light which the diamond 

 possessed, and ventured the opinion that it would 

 pro\-e to be combustible. Inconclusive experiments on 

 the subject were made by the older chemists, but the 

 true nature of diamonds was first discovered by 

 Lavoisier, a French chemist, who burnt them in oxygen, 

 and found they yielded carbon dioxide — the same 

 product as is obtained by the burning of charcoal. 

 A few years later it was found that graphite — the 

 substance from which pencils are made — when heated 

 strongly with nitre, was also transformed into carbon 

 dioxide. Diamond, charcoal, and graphite are there- 

 fore varieties of carbon, and are termed " allotropes." 



Allotropy ma}' be defined as the phenomenon of 

 the existence of elements in more than one modifica- 

 tion. However, not only elements, but also com- 

 pounds containing two or more elements, show a 

 similar disposition to exist in a variety of forms. 

 Calcium carbonate, for example, occurs in nature as 

 two completely different crystals, known respectively 

 as Iceland spar and aragonite. The term " Polymor- 

 phism " had been employed to characterise this 

 property of variation in the form of crystals of a 

 compound, and implies a property in compounds 

 similar to that which we have described in the case of 

 the element carbon. 



It has been known for many years that a variety 

 of oxygen can be obtained by the action of electric 

 discharges on the gas, which is endowed with much 

 greater chemical activity than ordinary oxygen. This 

 active variety of the gas is termed ozone. Ozone 

 differs from oxygen as regards the number of atoms 

 associated together to form a unit or molecule. The 

 molecule of oxygen consists of two atoms only, but 

 that of ozone contains three, and the greater energy 

 associated with the ozone molecule accounts for the 

 striking difference in its properties. An active variety 

 of nitrogen, very different in properties from ordinary 

 nitrogen, was prepared by Strutt about twelve years 

 ago. This active modification, which can be obtained 

 by subjecting ordinary nitrogen to the action of 

 electric discharges at low pressures, presents even more 

 striking contrasts, as regards chemical activity, with 

 ordinary nitrogen than exist between ozone and 

 ordinary oxygen. Ordinary gaseous nitrogen is an 

 extremely stable substance, requiring fairly high tem- 

 peratures to undergo interaction with other substances. 

 This great inactivity has been associated with the 

 remarkable stability of the two atoms of nitrogen 

 which constitute the molecule of the gas. The nature 

 of the molecule of active nitrogen is uncertain ; it 



may perhaps consist of a single atom ; in any case 

 it must be different from that of ordinary nitrogen. 



Results of Changes in Molecular Complexity 



^^'e must imagine the molecule of a gas — the least 

 quantity of a gas which can preserve the individuality 

 characteristic of the whole — as an association of 

 elementary atoms in a certain definite proportion. For 

 instance, di-nitrogen tetroxide consists of two atoms 

 of nitrogen associated with four atoms of oxygen to 

 form one unit or molecule of the gas. Now by raising 

 or lowering the temperature of this gas, we can make it 

 break up into two equal units of nitric peroxide, each 

 consisting of one atom of nitrogen and two atoms of 

 oxygen, or to unite again to form the original unit. This 

 change is associated with a remarkable change from 

 the colourless di-nitrogen tetroxide to the dark-brown 

 nitric peroxide. Similarly the unit of iodine up to a 

 temperature of 700° is composed of two atoms of the 

 element ; above that temperature the fine violet colour 

 gradually changes as the vapour dissociates into units 

 of one atom, the change being complete at about 

 1,700°. We see in these cases that a variation in the 

 number of identical atoms associated together in a 

 molecule — which we can refer to briefly as a change in 

 molecular complexity — is associated with remarkable 

 changes in the properties of the substances concerned. 



It has been considered legitimate to conclude that 

 the differences in properties of the solid and liquid 

 varieties of substances are also due to differences in 

 molecular complexity, since all well-investigated cases 

 which occur among gases can be explained in this way. 

 Our knowledge of the liquid and solid states of aggre- 

 gation is, however, very much less advanced than that 

 of the gaseous state. WTien, therefore, we say that 

 red and white phosphorus differ as regards their mole- 

 cular condition, we do not specify to what extent they 

 differ ; all that we mean is that the remarkable 

 difference in chemical reactivity exhibited by these 

 two varieties of phosphorus must receive an explana- 

 tion similar to that which accounts for the behaviour 

 of the varieties of substances which are met with in 

 the gaseous state. The crystalline condition of matter 

 is, however, receiving much attention at the present 

 time by the method of X-ray analysis developed by 

 the Braggs during the last few years, and results 

 obtained by this method upon polymorphic substances 

 will be awaited with great interest. 



Tlie Transition Point 



The temperature at which the solid and liquid phases 

 of a pure substance coexist in stable equilibrium is 

 termed the melting point. In the case of many 

 polymorphic substances (substances existing in more 



