350 PRINCIPLES OP CHEMISTRY 



part of it to charcoal at a relatively low temperature. For instance, 

 nitric acid when boiled with charcoal gives carbonic anhydride and 

 nitric peroxide. Sulphuric acid is reduced to sulphurous anhydride 

 when heated with carbon. When heated to redness charcoal ab- 

 sorbs oxygen from a large number of the oxides. Even such oxides 

 as those of sodium and potassium, when heated to redness, yield their 

 oxygen to charcoal although they do not part with it to hydrogen. 

 Only a few of the oxides, like silica (oxide of silicon) and lime (calcium 

 oxide) resist the reducing action of charcoal. Charcoal is capable of 

 changing its physical condition without undergoing any alteration in 

 its essential chemical properties that is, it passes into isomeric or olio- 

 tropic forms. The two other particular forms in which carbon appears 

 are the diamond and graphite. The identity of composition of these with 

 charcoal is proved by burning an equal quantity of all three separately 

 in oxygen (at a very high temperature), when each gives the same 

 quantity of carbonic anhydride namely, 12 parts of charcoal, diamond, 

 or graphite in a pure state, yield on burning 44 parts by weight of 

 carbonic anhydride. The physical properties present a marked con- 

 trast ; the densest sorts of charcoal have a density of only 1-9, whilst 

 the density of graphite is about 2-3, and that of the diamond 3-5. A 

 great many other properties depend on the density, for instance com- 

 bustibility. The lighter charcoal is, the more easily it burns ; graphite 

 burns with considerable difficulty even in oxygen, and the diamond 

 burns only in oxygen and at a very high temperature. On burning, 

 charcoal, the diamond, and graphite develop different quantities of heat, 

 One part by weight of wood charcoal converted by burning into 

 carbonic anhydride develops 8,080 heat units ; dense charcoal separated 

 in gas retorts develops 8,050 heat units ; natural graphite, 7,800 heat 

 units ; and the diamond 7,770. The greater the density the less the 

 heat evolved by the combustion of the carbon. 13 



By means of intense heat charcoal may be transformed into 

 graphite. If a charcoal rod 4 mm. in diameter and 5 mm. long be enclosed 

 in an exhausted receiver and the current from 600 Bunsen's elements, 

 placed in parallel series of 100, be passed through it, the charcoal 



15 When subjected to pressure, charcoal loses heat, hence the densest form stands to 

 the less dense as a solid to a liquid, or as a compound to an element. From this the 

 conclusion may be drawn that the molecules of graphite are more complex than those 

 of charcoal, and those of the diamond still more so. The specific heat shows the same 

 variation, and as we shall see further on, the increased complexity of a molecule leads to a 

 diminution of the specific heat. At ordinary temperatures the specific heat of charcoal is 

 0'24, graphite 0'20, the diamond 0'147. For retort carbon Le Chatelier (1893) found that 

 the product of the sp. heat and atomic weight varies, between and 250, according to 

 the formula: = T92 + 0-0077*, and between 250 and 1000, =3'54 + 0-00246 (see 

 Chapter XIV. Note 4). 



