348 



PRINCIPLES OP CHEMISTRY 



capability of absorbing the luminous rays (hence its blackness and 

 opacity), and many other qualities, are familiar from everyday ex- 

 perience. 11 The specific gravity of charcoal varies from 1*4 to l - 9, and 

 that it floats on water is due to the air contained in its pores. If 

 charcoal is reduced to a powder and moistened with spirit, it imme- 

 diately sinks in. water. It is infusible in the furnace and even at the 

 temperature of the oxyhydrogen flame. In the heat generated by 

 means of a powerful galvanic .current charcoal only softens but does not 

 completely melt, and on cooling it is found to have undergone a com- 

 plete change both in properties and appearance, and is more or less 

 transformed into graphite. The physical stability of charcoal is 

 without doubt allied to its chemical stability. It is evidently a 

 substance devoid of energy, for it is insoluble in all known liquids, 



a most marked degree. This property is particularly developed in that very fine and 

 friable charcoal prepared by heating animal substances such as hides and bones. The 

 absorptive power of charcoal for gases is similar to the condensation of gases in spongy 

 platinum. Here evidently there is a 6ase of the adherence of gases to a solid, precisely 

 as liquids have the property of adhering to various solids. One volume of charcoal will 

 absorb the following volumes of gases (charcoal is capable of absorbing an immense 

 amount of chlorine, almost equal to its own weight) : 



The quantity of gas absorbed by the charcoal increases with the pressure, and is 

 approximately, proportional to it. The quantity of heat given out by the absorption 

 nearly approaches that set free on dissolving, or passing into a liquid condition. 



Charcoal absorbs not only gases, but a number of other substances. For instance, 

 alcohol which contains disagreeably smelling fusel oil, on being mixed with charcoal or 

 filtered through it, loses most of the fusel oil. The practice of filtering substances 

 through charcoal in order to get rid of foreign matters is often applied in chemical and 

 manufacturing processes. Oils, spirits, various extracts, and vegetable and other solu- 

 tions are filtered through charcoal in order to purify them. The bleaching power of 

 charcoal maybe tested by using various coloured solutions such as aniline dyes, litmus, 

 &c. Charcoal, which has absorbed one substance to saturation is still capable of 

 absorbing certain other substances. Animal charcoal, produced in a very, finely-divided 

 state, especially by heating bones, makes the best sort for the purposes of absorption. 

 Bone charcoal is used in large quantities in sugar works for filtering syrups and all 

 saccharine solutions, in order to purify them, not only from colouring and odorous 

 matter, but also from the lime which is mixed with the syrups in order to render them less 

 unstable during boiling. The absorption of lime by animal charcoal depends, in all 

 probability, in a great degree on the mineral component parts of bone charcoal. 



11 Charcoal is a very bad conductor of heat, and therefore forms an excellent 

 insulator or packing to prevent the transmission of heat. A charcoal lining is often used 

 in 1 crucibles for heating many substances, as it does not melt and resists a far greater 

 beat than many other substances. 



