26 



KNOWLEDGE 



[Februaby 1, 1892. 



lampblack — in short, all the various forms of char — 

 appeared to possess iu an eminent degree what was termed 

 the " principle of combustibility/' The word Curhone 

 was emploj^ed to denote the combustible principle contained 

 in the various forms of char. The principle thus named 

 is represented iu terms of our modern views by what is 

 known as a chemical element, and to this element the 

 name Carbo/i is given. 



The phlogistic chemists made no real advance towards 

 explaining the reducing action of the various bodies con- 

 taining the principle or substance Carbon. This explana- 

 tion was reserved for Lavoisier, the great opponent of the 

 phlogistic system. Previous to the work of Lavoisier, Black 

 had prepared and studied the gas called "fixed air " (car- 

 bonic acid), which is obtained by the action of an acid on 

 Umestoue. Lavoisier found that the same gas is formed 

 when charcoal is burnt, and he fm;ther proved that iu this 

 process a constituent of the air (oxygen) unites with the 

 charcoal, the fixed air or carbonic acid formed being com- 

 posed of Carbon and oxygen. Lavoisier likewise proved 

 that in calcination the substance he termed oxygen is 

 abstracted from the air and combines with the metal, 

 forming a calx or oxide. Finally, he showed that if a calx 

 be heated with charcoal iu the absence of air carbonic 

 acid is formed, showing that the change which has taken 

 place consists in removing the substance oxygen from its 

 combination with the metal. 



From this series of experiments it was evident that the 

 redncinij fimction of any form of char is better expressed 

 by saying that the substance Carbon removes the substance 

 oxygen than by saying that the combustible principle 

 Carbon imparts to a calx the principle of combustibility. 

 These two modes of expressing the same facts may be 

 represented thus — 



Calx niinm the substance Oxygen = Metal, and 

 Calx plus the principle of combustibility = Metal. 



From the date of Lavoisier's discovery the formation of i 

 carbonic acid has been invariably employed for detecting , 

 the presence of the element Carbon. Carbonic acid gas is 

 readily recognized even in the smallest traces by the well- 

 known property of throwing down a sediment of chalk 

 when passed into lime water. Any substance which on 

 bm'ning forms carbonic acid contains, we say, the element i 

 Carbon. The amount of carbonic acid formed, which can ' 

 be accurately determined, supplies the best means of esti- 

 mating the weight of Carbon contained in any material. 

 If a material when completely burnt yields carbonic acid 

 and nothinij else then we say that the material is composed , 

 wholly of the element Carbon. The pin-est wood charcoal 

 when completely burnt yields no gas except carbonic acid, 

 and no residue except a very small quantity of ash in 

 which can be recognized those mineral ingredients which 

 the plant from which the charcoal was prepared originally 

 derived from the soil. Wood charcoal consists therefore 

 almost wholly of Carbon. Any other form of char 

 when freed as far as possible from foreign matter (<./;., by 

 drying to remove moisture) possesses the same, or nearly 

 the same, physical constants as wood charcoal. The specific 

 gravity is about 1-9. The materials are amori^hous, or 

 devoid of crystalline form. I 



It was not long after his discovery of the composition of ' 

 carbonic acid that Lavoisier, in conjunction with certain 

 other chemists, experimented on the combustion of the 

 diamond. The diamond was placed in a glass vessel 

 containing air, and the rays of the sun were concentrated 

 to a focus on the diamond by means of a powerful lens. 

 The diamond was by this means heated sufficiently to burn 

 in the air of the vessel, and the gas evolved was collected 

 over mercury, and tested. It was found to be carbonic acid. 



Soon afterwards (1796) an Enghsh chemist showed that 

 the amount of carbonic acid fonned by burning equal 

 weights of diamond and of charcoal is the same, a conclusion 

 which has since been repeatedly verified with the superior 

 accuracy of modern methods. 



Both diamond and charcoal are therefore said to consist 

 whoUy of the element Carbon. 



In the year 1800, a thhd substance was added to the 

 list of substances known as forms of Carbon. The mineral 

 plumbago, or graphite, was formerly regarded as identical 

 with molybdenum (a metallic sulphide), the appearance of 

 each being similar, and both possessing the property of 

 marking paper with a black streak, whence the name 

 graphite (ypu^o) to write). It was shown, however, by 

 Mackenzie, that graphite burns with formation of carbonic 

 acid, the amoimt formed from a given weight of the 

 material being the same in the case of charcoal and of 

 diamond. Here, then, we have a thu'd form of Carbon. 



These three substances differ, in the first place, in certain 

 important physical characters. The specific gravities are 

 different, diamond standing highest in the list, and charcoal 

 lowest. Diamond crystallizes in the regular or cubic 

 system, graphite iu the hexagonal system, whilst charcoal 

 has no crj'stalline form or structure. 



But it is not only in physical properties that the three 

 substances difi'er ; they differ to a certain extent also in 

 chemical character. The temperature at which diamond 

 burns is much higher than that at which the combustion 

 of charcoal takes place. Thus, although oxygen unites 

 with either substance, and iu each case forms the same 

 product, yet the readiness with which this combination 

 takes place is very different in the two cases. In other 

 words, the chemical relations of diamond and of charcoal, 

 with respect to oxygen, are by no means identical. Again, 

 graphite differs from either of the foregoing, in that the 

 combined action of nitric acid and potassium chlorate 

 convert it into a peculiar acid, a solid substance, known as 

 graphitic acid. 



In spite of these marked differences, chemical as well as 

 physical, it is the universal practice to denominate all 

 three substances, diamond, graphite, and charcoal, as 

 " forms of the element Carbon," or aliotropic modifications 

 of Carbon. It must be confessed that the present phrase- 

 ology is not as clear as might be desired, and is constantly 

 a stumbling-block in the way of the tyro in chemistry. 

 Set a schoolboy to write an essay on " allotropy " (or the 

 existence of elementary substances in different "forms"), 

 and he will choose as his example the element Carbon. 

 He will begki by poiatiug out how widely different are the 

 substances diamond, graphite, and charcoal, and will wind 

 up his essay by saying that, notwithstanding these differ- 

 ences, " they are really the same thing — Carbon." 



The source of confusion must be explained bj' reference 

 to the atomic theory. We possess a great mass of evidence 

 to show that what we observe in any chemical process is 

 in fact the sum total as observed on the large scale of a 

 number of phenomena, all precisely alike, occurring 

 between the itltimate particles or chemical atoms of the 

 substances. All the thousands of known substances are 

 formed by various combinations of atoms of a comparatively 

 small number of chemical elements. All the atoms of any 

 one chemical element are exactly alike, but are different 

 (f._</. in their mass I from the atom of any other element. 

 A substance containing more than one kind of chemical 

 atom is termed a compound substance or chemical 

 compound. A substance containing only one kind of 

 chemical atom is termed an elementary substance. Such 

 a substance is diamond ; it is formed wholly from atoms 

 of one kind, from Carbon atoms. Charcoal likewise is 



